Interbreeding and speciation are pretty confusing.
Tiger and lions are different species, yet they can interbreed, AND not all hybrid products are sterile (look up liligers). I think female ligers are actually fertile. Yet, tigers and lions are still separate species.
That's beside the point, though. Yes, the "races" of WoW are technically species, but that's a semantic issue and no one really cares at this point. And, you could make an argument that being in a fantasy universe mucks things up a bit.