That is an interesting theory. I have also held that the antediluvians were much more advanced than modern man (man has been de-evolving since the first man, Adam, and a 900 year healthy lifespan added to their capabilities); and that there must have been some pretty 'bad stuff' going on that caused God to wipe out all but 8 people from the earth. As well as all land animals, besides the ones on the Ark. What it all was, I don't know, but your theory is plausible.
I still think that the Neanderthals, though, were human - just towards the Offensive Lineman scale of human builds.
Ross has, I believe, pointed out a significant genetic difference between the two physical lines. They might have been tempting fornication partners but wouldn’t have resulted in any offspring.