Join Date: Aug 2009
Location: Portland, Oregon
Several reasons come to mind.
In universe? Betrayal is written from a post-Heresy chronology. Some record of their gene-seed perfection could have been lost.
The author may have wanted to avoid writing on the perfection of their gene-seed because clearly it was not--their loyalty ultimately failed. How could their gene-seed have been perfect if they betrayed the Emperor?
Out of universe? As I said before, there's a lot to stuff in there. Maybe for purposes of space and brevity, they skipped over this fact.
Maybe the author writing the book didn't read all of the Index Astartes and forgot to add it. Maybe the author didn't like it or felt it wasn't important enough to add.
The lack of evidence isn't proof enough to discount something. Re-reading the Emperor's Children portion of the book, I don't think I saw any mention of Fulgrim and Ferrus's close relationship. Should we write that off, too?