> sounds all very unsubstantiated. Speculation it seems to me

What part of the study strikes you as unsubstantiated?

Every part is unsubstantiated. For starters, for the vast majority of H. Sapiens existence on earth--from 300,000 years ago to about 45,000 years ago, we shared the world with 4 or 5 other hominids that we know about. (Neanderthal, Denisoven, H. Luzonensis, H. Floresiensis, and still perhaps a few H. Erectus, and no doubt even more we haven't found yet.)

That's 250,000 years of coexistence. We know that we sheboinked with at least two other species, probably more, because we still carry their genes to this day. So much so that it couldn't have been just a sheboink or two; we sheboinked over extended periods of time, i.e. we formed families with Neanderthals and Denisovens.

We have no evidence of warfare between the species. I.E. We have found no Neanderthal skull with an arrowhead in it, for example. Besides the fact that we are the only ones left, I don't see any substantiation at all.

It is a mystery why they are not still here. But the last 50,000 years, since the end of the last Ice Age, has been very hard on human species, for some reason. We are the only humans left, what every got them might get us too if we let it.