This is one of the subtle testimonies of the truthfulness of this record. Had Joseph Smith simply been creating the Book of Mormon, fabricating it (rather than translating it), he probably would not have inserted such ideas into the narrative.
Here we see that Mormon, a powerful prophet-editor, was simply unaware of what became of Hagoth and his followers. Living almost five centuries after their departure from the promised land, Mormon could have inquired as to their whereabouts, but presumably he had not done so, or if he had, he had not learned by revelation what became of those people.