I understand the science behind it. I know all about neanderthal and its genetic distribution. I have read about neanderthals' slight affects on things like back hair (less interestingly enough) and circadian rhythms. (Interesting note, I have more neanderthal DNA than 99.9% of people. )Thing is, irl many different species can interbreed to produce fertile offspring. Our own species interbred with at least two other species in the past, which is why some modern humans have DNA from neanderthals and/or denisovans.
That is not what I am referring to when I am discussing fantasy games or Wizard's possible take on it. There is a lot of handwaving when it comes to culture, species, etc. in fantasy. This is one of them. Just say they are species that don't produce offspring. There are many examples of that in nature too. It is literally possible to just choose one, since as the game creator, they get to play, not just God of creation, but also God of science and magic.