As a counterpoint that would imply wolves, coyote and dogs, donkeys and horses, lions and tigers, house cats and bobcats, grizzly bears and polar bears, etc. are all the same species, because they are all capable of interbreeding with viable offspring.

Taxonomy is not a settled question, and I'm not trying to imply it is. But neanderthals exhibited significantly different traits from humans. There is probably a case to be made that modern homo sapiens are a subspecies of neanderthals and another hominid. Hybridization can (and frequently does) result in new species distinct from the forebear species, after many generations. But speciation doesn't preclude interbreeding.