Geography: This is a new story, telling the story of the contact between the Lamanites and Almaites from a different perspective, that of the Lamanite army.
This is the very army that has been following Limhi's people. We learn that they were "lost in the wilderness for many days." This confirms both the presence of the strip of wilderness between the land of Nephi and the land of Zarahemla, but confirms the difficulty of finding a way though that wilderness to Zarahemla.
There is certainly no clear trail, no well-traveled trade route, between the two locations. This is a wilderness sufficiently difficult to traverse that Limhi's group could be lost in it and miss Zarahemla, the Lamanite army could lose the trail of a large number of people fleeing with animals and goods, and they could "stumble" onto a settlement in a mountain valley that they apparently did not know was there, and certainly did not know had been inhabited for a year or more.
Textual: Mormon uses the word "now" as a verbal marker to begin a new subject, a process we have seen in several other occasions. We cannot be certain which records Mormon is using to relate this story, but the best guess would be that this information was made known to Alma's people, and came to be recorded with Alma's official record which Mormon clearly has before him.