I can see how that would be puzzling.
With this and the Hobbit discoveries, I think that there were quite a few more unknown ancient human species running around the planet at the same time that our direct ancestors were here than we now know about.
It seems they left out another possible solution, which is that the ancestor of the Australoid aborigines bred with the Denisovans BEFORE the Australoids crossed the Wallace line.
We KNOW the Australiods did—that is why they’re found in Austraila, New Guinea, the Solomons, etc. They had to have used boats to get there in the first place.
East Asians moved from the interior of Asia into Southeast Asia at the glacial maximum—in the Sundaland and East China land masses that are now underwater. They drove the Australiods into Australia.
The racial characteristics of East Asians—thick hair, eye folds, paler skin color, seem to point to an origination in a dry, cold sunless climate, where the Australiods originated in a hot, sunny area. In other words, East Asians came from the far North and displaced the Australiods living in Southeast Asia and Indonesia.
I've read that the dna from this little piece of bone has had a more thorough dna analysis than any other bone anywhere.