When Mormon declares that Moroni “named all the land which was south of the land Desolation … a chosen land, and the land of liberty,” he was describing the land of Zarahemla. Moroni would have had no intention of suggesting that the lands of the Lamanites to the south of the land of Zarahemla were a “land of liberty.” As noted in the comments on verses 17–18, Moroni used the concept of “liberty” to mean liberty to believe in the Nephite religion. It was not a declaration of a political system except as that political system was founded in the Nephite religious ideas of egalitarianism.
Moroni is using different language to reinforce the covenant of the land. He was declaring that the covenant applied to the land of Zarahemla and to those who believed in the Nephite religion (specifically those who had taken upon themselves the name of Christ) and that God would support them as they combated this threat.