Rhetoric: After reminding them of Lehi’s promise, Alma’s next device is to hold up the Lamanites as the bad example of Lehi’s descendants, in contrast to the privileged and promised Nephites. This line of argument tells us that, while the Ammonihahites did not consider themselves as wholly Nephite (witnessed by their rejection of Alma as the religious representative of Nephite culture), neither did they consider themselves Lamanite. Therefore, the source of the Nehorite florescence was not Lamanite.
Alma is assuming that his listeners, who are culturally Nephite, will automatically feel a cultural dislike of Lamanites. He begins by simply asserting what his listeners already understand: that the Lamanites are cut off from Yahweh’s presence. He would not have made this argument if the Ammonihahites had not already accepted it implicitly.