I don't believe that. How hard is it to capture baby horses, it would happen naturally hunting their parents. Horses bond decently with people and if captured and socialized heavily by the tribe from a young age I have no doubt first generation wild horses would be rideable and could be bred.
Wild horses run in herds, I think they're naturally social. If anything, it takes less work now - ranchers can ride horses that get very limited human contact, but the I image first generation tamed horses must have required a lot of socialization.
