This verse is the temporal equivalent of Mosiah 23:1:
"Now Alma, having been warned of the Lord that the armies of king Noah would come upon them, and having made it known to his people, therefore they gathered together their flocks, and took of their grain, and departed into the wilderness before the armies of king Noah."
Each tells precisely the same events and story. Mosiah 23:1 must repeat this information because is it physically removed from the story, and there is the insertion of a different story (that of Limhi) which will move to its completion before Mormon returns to Alma and his people.