Why do we have to posit non-human monsters as explanations for these myths? After all, there were (and still are) real monsters out there that would kill, rape, kidnap and torture people - people from other communities. Strange danger was serious business back in the day, why could that not evolve into all kinds of boogeymen?
This is what I thought when was evoked cannibalism: what if those were the traces of sapiens festing of neanderthalis (and not neanderthalis eating neanderthalis, as seems to be supposed in the text)?
Another point seemed to me a bit far fetched: that due to anatomy Neanderthals could not speak. But there are birds that imitate pretty well human speech.