Moroni had invoked God as the author of their victory, but Zerahemnah does not see the hand of God in the battle. Of course he knows nothing of the intelligence about the attack on Manti that Alma received from the Lord. What Zerahemnah did understand was that Moroni had used unfamiliar tactics (“your cunning”) and had provided armor that had made a difference.
The reason Moroni had invoked God was to indicate that the outcome would be the same in any future engagement, so the Lamanites might as well swear not to fight. However, Zerahemnah does not recognize God in the process, so he does not accept the inevitability of future similar loses. Zerahemnah is willing to surrender the present, but not the future.