Probably some did breed with the lesser humanoids but those offspring probably didn't survive...unless you count some of the folks here who, oh never mind.
I’ve had my DNA tested, and I’m 3.2% Neanderthal. I have more Neanderthal DNA than 96% of the population. And what’s funny is, all my life I joked that I was part caveman (well, cavewoman.) As it turns out, I am.
I think the interbreeding theory is extremely unlikely. It would require that the 2 groups co-habitate and that there be virtually no preference for mating within one’s group rather than with the other. This simply doesnt happen with primates or other territorial animals. It also requires that the offspring of such a mating are fertile which is also unlikely. It is much more likely that one group wiped out the other either through disease, aggression, or outcompeting them for resources. Even if the 2 groups were equally intelligent, a difference in the level of aggression of one culture relative to the other is enough to cause extinction of the less aggressive group.