Moroni takes advantage of his advance information to lay a trap for the Lamanite armies. This valley is certainly part of the access lines to Manti. Knowing their intended target, Moroni deploys his forces to intercept them prior to reaching their target. What is interesting here is that while the Lamanites clearly have a head start from Jershon to Manti, Moroni arrives before they do, and has time to set the ambush. He is able to do this even though he had to send messengers to Alma, have Alma inquire of the Lord, and then have those messengers return with the information.
This sequencing of events suggests that Moroni’s line of march from Jershon to Manti was much shorter than that of the Lamanites, precisely the situation expected because of the natural barriers to the west of the Lamanite armies. As they retreated and moved around the wilderness area, they took much longer than Moroni’s men took as he had the more direct and topographically easier route up the Sidon river valley.