Social: Here is the most explicit correlation between Alma's baptism and the covenantal entry into a subset of society. To be very clear, Alma is preaching to Limhi's people who have been with Ammon for some length of time (perhaps a year?). This is a people who have repented of their ways, and certainly would have returned to a belief in God as understood and taught by Ammon. Thus Alma is not baptizing unbelievers, but believers.
While it is tempting to suppose that this is a baptism required to return the Limhites to the Nephite religion, this begs the question of Ammon and his possible authority to baptize. Clearly Ammon did not baptize. This ordinance waited for Alma, just as those who left Lehi-Nephi to go with Alma were baptized in the waters of Mormon.
The verse very clearly notes that "as many as he did baptize did belong to the church of God." Once again we see this conception of "church." For the first time we have an explicit demarcation in Nephite society between a congregation of God and those who were not.