Going back over Humaniti, that book has it as speculation rather than fact (and oddly put it in Physiology and Appearance rather than Origins and History). The chapter does state that scientists speculate they might be "completely artificial genetic constructs," but doesn't contain a statement on whether that speculation is accurate. It also says that their origin is unclear and "biological evidence indicates" they were subjected to substantial genetic manipulation by the Ancients, so we end up with a narrator statement that their origin is unclear, that there's evidence of Ancient genetic manipulation, and scientists speculate they may be Androids, but other than the "origin unclear" statement they're not phrased as unambiguous fact. It's not quite as fuzzy as T4 canon, but it's not a direct statement of fact either.
(The Physiology and Appearance section also claims they're not hominids, which would mean they technically don't belong in the book, since per Humaniti "n the Imperium, the collective term for all intelligent hominids is Humaniti." If they're not hominids, then they're not part of Humaniti by the Imperial definition. It would also mean they're not Homo anything, since when that book was written, all of Homo was contained within hominidae - it's since expanded to also contain orangutans, gorillas, chimpanzees, and bonobos, but I think that was still contentious when Humaniti was published.)