Mormon must have had a laconic sense of humor. It is virtually certain that this description of Ammon as "wise, yet harmless" is Mormon's and not part of the text that Mormon is copying. Why would Mormon insert this phrase, or that Lamoni was "caught with guile?" Why these terms?
In a sense, Ammon did trick Lamoni, though benevolently, and appropriately for his mission. Lamoni had offered material goods, either protection, or implicit wealth. Ammon asks for none of these material things, but rather a very simple request that the king listen to him preach. Surely Ammon understood that the offer was for material goods. What Ammon does is use the traditional inviolability of the word of a king to request the chance to preach. Lamoni could not refuse Ammon's offer. The wry humor in the situation is Mormon's understanding of the nature of the trade Ammon made. The king had offered tremendous wealth and perhaps power, and Ammon took only the chance to make a speech. What Mormon also knows is the result of Ammon's preaching, and that what Lamoni eventually paid was his soul, but a payment not to Ammon, but to God.