Verse 7 sounds as if there were more than twenty-four survivors. Even though that was the count Moroni gave, it would not be surprising that there were some who claimed to be Nephites who had not participated in the battle, but remained in smaller villages that had been passed by. Those were also hunted down, presumably so that they would not hold a grudge and some future day be strong enough to act on it.
Verse 9 tells us that Moroni followed his father’s descriptions of the nature of the people who had destroyed the Nephites. It was not just Lamanites, but Lamanites and Gadianton robbers. At the end, there were only the Lamanites (the traditional southern enemies) and the Gadiantons (the more recent northern enemies).