I thought it was shown that early humans interbred with various humanoid species? Where does the "weakly interfertile" assertion come from? (Not saying its not true, just that I never heard it before)
At the point where we have separate species, we have evidence of intermixing, but it's somewhat weak (about 5% in populations which would have lived next to Neanderthals).
Basically like dolphins today; they can almost all interbreed and are even second-generation fertile, but their overall fertility is depressed relative to same-species breeding and their social institutions/environmenal niches keep them separate enough to maintain that.
Same as early modern humans and their contemporaries, basically. Except for the whole part where we (probably) killed them off (unlike dolphins who only impact each other's populations slightly).