First of all, everybody knows the origin of black Americans. As for Newars, their origin is shrouded in mystery. To use your logic, just because black Americans speak English as their native tongue, we cannot jump to conclusions and say that they're Anglo-Saxons. So, I would not rely solely on language to conclude that Newars are of Tibetan origin. Newars are just a group of people that comprise both the Tibetan and Indian peoples. Newari is not purely Sino-Tibetan; in fact it is mixed with Indian dialects. Those Hindu Newars were not converted to Hinduism. They were Hindus for generations. How exactly are you going to explain why some Newars look no different from brahmins and chettris? Newar Brahmins settled in Nepal earlier. Those that came later on did not accept them as brahmin.