Mormon here reveals a dry sense of humor. This description of Ammon as “wise, yet harmless” is almost certainly his, not part of Alma’s record and certainly not Ammon’s account of his mission to Alma. Why would Mormon insert this phrase or explain that Lamoni was “caught with guile?”
Surely Ammon understands that Lamoni is offering material benefits (protection, wealth, and possibly that daughter again), but Ammon asks only that Lamoni listen to him. Lamoni, having offered so much, cannot refuse this lesser request. Trapped by the traditional inviolability of a king’s word to request the chance to preach, Lamoni has become Ammon’s captive audience. The humor lies in the fact that Mormon knows the outcome. What Lamoni eventually paid was his throne, his homeland, and his soul—a payment not to Ammon but to God.