Overall, I think there's a little miscommunication going on here on all sides. It lies somewhere between what we know and what that actually amounts to when expressed in a story.
Lytek can't restore an Abyssal Exaltation, but he can restore an Infernal one. This goes back to an issue of frequency. In the case of Abyssals, the frequency of the Solar Exaltation has been changed beyond his power to do anything about it. This is a very underboard way of saying it has been deeply tainted and inverted in such a way that is has not been fundamentally changed, only warped, and it is beyond his ability to restore it, as he cannot influence Exaltations internally to change how they function. Infernal Exaltations are just frontloading a ton of Primordial charm technology, mutations if you will, that he can scrape off the Exaltation and restore it. It is surface-level change. Infernal Exaltation similar to just wrapping a perishable temporary Exaltation over the imperishable Solar Exaltation, so it's kind of using the Solar "shard" as a battery pack.
What I think Maddy is trying to point out is that it's an artificial way of stopping the Unconquered Sun from changing an Infernal, which is a wholly original Exalt from the Solars, into a Solar. Infernals are as unique from Solars as any other Exalt type except Abyssals. I do think it is a rather arbitrary way of keeping Infernals inviolate within the setting, seeing as their basis begins with a Solar Exaltation, but it is wholly understandable. It is to prevent one entirely different Exalt from becoming another absolutely different Exalted. The Unconquered Sun can't lift the condition because the person has been changed on a deeper metaphysical level than just a pursuit of the light and goodness can overcome. Light and goodness, ergo, are not contrary to Infernal Exaltation in a way which forms a metaphysical solution to the condition. So a redemption that ends with the Infernal becoming a Solar isn't possible.
Before anyone makes the mistake of thinking I don't understand the setting logic for this, I do. Do you? Abyssals and Solars are a matter of transmission. They are two halves of the same coin, counterpart Exalted. In a word, the tragic fall to an Abyssal Exalted, or the heroic, arduous rise to a Solar Exalted, was something built into these Exalts as a matter of fact; of being able to tell the absolute pinnacle standby story of falling to the furthest depths of darkness or fighting your way back to light. That is not a theme that is true of Infernals.
Solar <-> Abyssal (true)
Solar -> Infernal (true)
Solar <- Infernal (false)
It is valid to characterize Infernal power as negative. It's tainted, demonic, inherently heinous, and it comes from an unclean and unholy source. It is valid that an Infernal Exalted might seek the light of redemption to be free of the Yozi disease, because she is so disgusted by or disturbed by the terrible taint of her powers, but it would be doing Infernals a disservice if any path they take led them to becoming Solars. It sort of undercuts their mission and their thematic impact to even imply that they need to be Solars, in spite of whatever in-game reasoning there would be. It cuts the weight of the condition if, after some long transcendental quest, you could holy your way back into the Unconquered Sun's good graces and it resulted in you becoming a Solar Exalted, as if that were a superior state of being. Yes, for the purpose of being rid of your evil alien hellish connotations, it is, but the actual idea that you can fundamentally tear apart an Infernal Exalted and reach the Solar underneath goes against the entire metaphysical structure of what they are from top to bottom. It makes their Exaltation a bit less legitimate.
I think the best you can hope for in terms of Infernal redemption, is having your individual CoD status lifted by the Unconquered Sun. But that would take a Hell of a lot of work and evolution on your part. Fortunately, Infernals are more free than Abyssals to pursue this storyline.
So it's not that Infernals can't seek redemption, or that the idea is invalid. They are quite monstrous and unnatural. It's just that you can't pick someone up who hasn't fallen. They aren't coming to the setting from a basis of returning to Solar status because they are, inspite of their historical basis, a wholly different Exalt from Solars.
Seriously though, redemption doesn't have to mean a transition into a Solar.
I think when you begin to mince meanings and come up with different degrees of damnation, the whole theatre of the discussion has gotten a little bit silly. Infernals are an impure source that grates against the core; they are damned, and the Unconquered Sun, an honorable being, might not smite one who comes to him looking for aid, but he would turn him away with words that would chill the blood ice cold. What could possibly be worse than being turned away from the light of the Sun with the knowledge that you cannot be helped? In fact, you can—but just sit with me in this instance for just a moment longer, if you will? How much more condemned can you possibly get than being told by the Unconquered Sun that you are alone in this?
However, we know, or at least it is my belief that, the UCS can offer deliverence to an Infernal. It is just not the sort that terminates in a Solar Exaltation. Yet an Abyssal can attain a redemption that besets this tremendous metaphysical change, and he can do so from the outset. So why are Abyssals "more damned" than Infernals? Well, for one thing, Abyssals have had to throw their names away and admit that death and oblivion are supreme and inexorable and that this is the only path for Creation and existence. They have, in essence, been made slaves to the most vile, innimical forces in all of the universe. Their redemption, then, is far more unlikely than an Infernal's.
Another thing to consider is that there is very much a bone of contention between Heaven and Hell as to who owns Creation. Even imprisoned, the Yozis metaphysical-legal authority over Creation is so great that you can see the beginnings of a contest over who will own the universe when all is said and done. Just because they sit in prison, don't think the score has been settled on this issue. Infernal Exalted further represent that the Yozis have a dog in this fight, out there enforcing their metaphysical primacy and legitimacy, and ardently seeking their parole. And there has been more than one suggestion that, though Yozis are damned, twisted, demented and driven down, the gods victory over them and the Exalted strictures' over them are not one hundred percent binding, legal, or just. The Yozis yet retain some authority over existence, and the means to take it back. They are blasphemous and condemned, but the fact of their legal presence prevents a wholesale rejection. It is not a matter that can be settled by pure denial and reactional outrage to their existence, because such a feeling does not carry metaphysical weight enough to lay such a heavy spiritual condemnation on their heads. They have far too much relevance.
The Neverborn, however, are dead and blasphemous to a degree that even the Yozis are sickened and disturbed by them. The Neverborn have lost their relevance, and all they have is something that makes absolutely no sense to the Yozis, and which cannot and must not be imparted upon Creation: the finality of all-encompassing death. In essence, Abyssals are "more damned" because the hammer swings much harder and farther against them from all corners of the universe. An Infernal may be alien, demonic, monstrous, and bad, but even he may look upon an Abyssal and feel that cold, sinking feeling as he senses his own end coming out of that thing.
Not to reduce the subject to something oversimple—because it is a complex topic worthy of part-by-part examination—I am nonetheless going to present a visual aid. This is not meant to be the be-all, end-all criteria, it is merely an example. In fact I realize that such an example robs the whole issue of complexity, so it is not meant to be the way by which you should judge it—I just feel it proffers a helpful example of how to gauge this issue. And here's where it gets extra silly, but I am being serious, even as I am being humorous. It goes something like this:
Picture your place on the scale of damnation being how disturbing you are to the Unconquered Sun.
-Not disturbing at all
-The Maidens
-Luna
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-Raksha
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-People dressed as cupcakes
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-The Yozis (Infernals)
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-The Neverborn (Abyssals)