The description of voluntary submission is very much in the nature of city-state/tributary relations. The all important goal was the establishment of the tributary relationship (Ross Hassig. Aztec Warfare. University of Oklahoma Press, Norman, 1988, pp.105). A city that still had the capabilities of production made for a much more profitable tributary. The relationship was not one of slavery, as the city could typically retain its own government:
“Governance of these conquered areas was tailored to the hegemonic imperial system: the Aztecs were unconcerned about many local activities. The allowed local laws, customs, and beliefs that did not obstruct imperial aims to be retained, even when they differend from those of the Aztecs. Thus incorporation into the Aztec Empire did not necessarily mean that tributaries altered their behavior vis-à-vis other subgroups.” (Ross Hassig. Aztec Warfare. University of Oklahoma Press, Norman, 1988, pp.21).
When Giddianhi suggests that the Nephites join with them, it is a sincere offer of membership in the city-state’s political umbrella. Of course it would still be membership as a tributary, but it was membership nevertheless, with the retention of current social positions among the Nephites.