The definitional limit on interbreeding includes geography and preference; black/brown/polar bears can and do interbreed in captivity but because they live in different locations and they look sufficiently different/have different diets we consider them different species.

If Aliens landed on our earth 20k years ago and used our definition of the word there's no doubt in my mind they'd consider us different species, we'd be like bears. As far as I can tell the only reason we don't classify people as different species is for the sake of society, as people would naturally then try to determine differences and rank them, which has an obviously bad long-term outcome.