Well, clearly that was not true, because there are billions of people today who have neanderthal DNA (in fact, almost all of us do according to one study), so clearly those offspring were not sterile.
We do have examples of that here in the game, while they are not sterile, most of the drow/human hybrids have major disabilities.
Now that I think about it, maybe with some genetic engineering, a drow human hybrid could be made safely... We need a geneticist in the harem anyway, how else will we use the elf DNA to make the MC and his girls live longer?!?!