In addition to the non-Aztec natives who allied with the Spanish, the GREAT DIFFERENCE was technology. The Aztecs were living in what was essentially a Stone Age material culture. The Spanish brought, among other things, weapons, iron, steel, metal armor, the wheel and horses as well.
Yet, they were otherwise so massively outnumbered that the steel and firearms would have made little difference. I think the native help used to be discounted in order to enhance the glory of the feat and recently it has been ignored in order to demonize the feat.