Mormon has been paying more attention to the north, and he continues that trend in these verses. Without giving much information, other than the brief notice here, he notes that “five thousand and four hundred men, with their wives and their children” went north. That is a very large number of people. We will see more northward migrations early in the book of Helaman.
Mormon also briefly relates the story of Hagoth. He helps many migrate by sea, rather than by land. Although Mormon doesn’t say where they went, he notes that they launch near the narrow neck leading into the land northward. Perhaps he was hinting that they, as well as the five thousand four hundred men (and their families) had gone north.
For Mormon’s purposes, it is sufficient that he document the movement of people who were Nephites toward the more northern lands. He will be even more explicit, starting in Helaman 3:3.