The story of Aaron’s missionary labors obviously began when he, Ammon, and the others separated and went different ways. That event was recorded in Ammon’s story (see Alma 17:17). It is told in less detail here, not because Aaron didn’t say anything about it, but because Alma knew he had already described it. The mention here is to situate this story as happening parallel to Ammon’s story. Therefore, Alma notes that they start at the same time.
Aaron has a different experience because he goes not to just Lamanites, but also to apostate Nephites who had become Lamanites. The Amalekites were dissenters from the Nephites, and the people of Amulon followed the man who was once priest under King Noah. Mormon will consistently paint the Lamanites as enemies, but will just as consistently suggest that the worst enemies were those who had once been Nephites and consciously turned their backs on Nephite beliefs.