Even though the Lamanites had become more cautious of strategy, they see a very weak army encamped about them, and they see an easy victory close at hand. They do have some concern because of the supply route. They know that the disruption of their supplies could hamper even an army of their size. Therefore, they consider this just one more tactical sally, but on a larger scale. Possessed of apparently overwhelming numbers, they presume that they can destroy Helaman’s army with a finality that would likely give them final victory in this area.
The reason that this strategy works is that Gid and Teomner depart with their men only after the Lamanite spies have reported the troop strength. Those spies had certainly ascertained the total Nephite troop strength, and when they returned to report, undoubtedly indicated that the entire Nephite army was camped around Manti. With the whole of the army in front of them, the Lamanites have no hint of a trap, because they do not believe that there are any more men in the region who could spring a trap. Indeed, they were actually quite correct in this, for Gid and Temoner’s men were with the main army when the spies counted. They left after the count to set the trap.