It is unlikely that this justification for Chief Captain Moroni’s actions were on the large plates. It is an interesting view into the mind of a military man that Mormon needs to justify Moroni’s action of defending the Nephites by a stratagem. Mormon implies that such tactics might not be completely honorable. Nevertheless, because the intent behind them was honorable, Mormon sees the stratagem as justified.
The only reason for the inclusion of these two verses in an otherwise straightforward telling of the military situation is that Mormon had some level of discomfort with the idea of defeating an enemy by essentially using trickery rather than what he must have felt was a more honorable face-to-face conflict.