I didn't actually specify a reason why the 4MM Form doesn't include soulsteel, I think. :)
Many possibilities occurred to me, so I decided it was more interesting than mechanical completeness; but I don't think that one can conclude with any certainty that soulsteel itself postdates the form. Personally, I think it's more interesting if the Sidereals designing the style left it out - either because they had to, because they screwed up, or because they chose not to include it.
Can you think of reasons for any of the above? :)