This verse is conceptually similar to Jacob’s discussion of who was a Nephite and who was a Lamanite:
Jacob 1:13-14 Now the people which were not Lamanites were Nephites; nevertheless, they were called Nephites, Jacobites, Josephites, Zoramites, Lamanites, Lemuelites, and Ishmaelites. 14 But I, Jacob, shall not hereafter distinguish them by these names, but I shall call them Lamanites that seek to destroy the people of Nephi, and those who are friendly to Nephi I shall call Nephites, or the people of Nephi, according to the reigns of the kings.
In both cases, we have a separation of the label from the lineal descendents. Even in Jacob, the people who are Nephites are those friendly to Nephi – and those who follow the reigns of the kings who were named Nephi.
Mormon tells us that the same type of designation served throughout the Book of Mormon. Those who believed in the records and the traditions of the Nephites were called Nephites. While Mormon gives a religious definition as well (“who believed in the commandments of God and kept them”) but we must continually remember that religion and politics were all rolled into the same concepts in the ancient world.
We should note also the dichotomous relationship between the categories of Nephite and Lamanite. For Jacob, those who opposed the Nephites were Lamanites. In Mormon’s statement, those who weren’t Lamanites were Nephites. Both prophets give only two categories, Nephite and Lamanite. Neither men, however, link those labels to lineage, as we might otherwise expect. These are designations that are broader than a kin group. They define a poltico-religious body.