Moroni uses the captured Lamanites as forced labor in fortifying Bountiful. Mormon feels some need to explain this use of Lamanite labor, suggesting that the practice was uncommon or even that it might reflect badly, in some way, on Moroni without the explanation.
The Nephite armies, fewer than the Lamanites to begin with, are split into two theaters of war. The Nephites who would have been available to labor on the fortifications are already in the army. The Lamanites provide a ready work force that must also be fed (and therefore might as well earn their keep), in addition to the practical consideration that it was easier to guard them with a few men while they worked than to detach even more men from the army to guard duty.
As a result, Bountiful “became an exceeding stronghold ever after.” As I read this description, Bountiful seemed to have continued as a fortified Nephite city up to Mormon’s day.
Translation: Here is another case of an inserted correction; and as usual, either Mormon or Joseph may be its author. I see Mormon as the more probable author of the insertion because the writer had to clearly and simultaneously understand the differentiation between the land of Bountiful and the city of Bountiful. It was obviously impossible to create such extensive defenses for the entire land of Bountiful, hence the rapid correction to the city. It seems less likely that Joseph would have been as keenly aware of the land/city difference.