Biologically speaking, Neanderthals were humans.
If "modern humans" and Neanderthals could have offspring which were able to have children of their own, doesn't that mean that they belonged to the same species?
It's not new that there was mating between modern humans and Neanderthals, but this study may put it at a much later date than had been guessed earlier.
The children resulting from such matings must have been accepted as members of the group by early modern humans--they weren't shunned.