The classification of species needs some work. Just because two populations can occasionally mate and produce fertile offspring does’t mean they are always classified as the same specie. If there are limits to their ability to produce fertile offspring - for example only offspring of one sex can be fertile and the other usually sterile, or the offspring can occasionally be fertile but most often are not, or even if the offsprings are fertile but only with one of the two parent populations, then the parent populations would usually not be classified as the same species.
How readily different populations of archaic humans can interbreed and produce offsprings is AFAIK not well known, All we know is it can happen. But how readily that can happen has a lot to say about whether they are the same species.
How readily different populations of archaic humans can interbreed and produce offsprings is AFAIK not well known, All we know is it can happen. But how readily that can happen has a lot to say about whether they are the same species.