BTW is there a good comparison for how big the anatomical differences are between Neanderthals and modern Europeans, compared to the differences between modern Europeans and modern, significantly distinct populations such as Congo pygmies?
https://en.wikipedia.org/wiki/Congo_Pygmies
(Anatomic differences are perhaps not as important as cognitive and psychological differences, but the latter are very hard to find out about extinct populations.)
Yes there is no one universally agreed definition of species, but Neanderthals were vastly more different from living humans than and differences between living human populations.