Ammoron dismisses Moroni’s threats. Very likely they were standard diplomatic gambits, despite the anger each expressed, which was probably genuine.
In spite of the bluster in the reply, in which he fully matches Moroni’s own bluster, Ammoron readily agrees to the exchange. Despite Ammoron’s strong position, his supply line is long, and it is not surprising that he would want to be conservative with the food available. By exchanging at least three to one, he gains military men and frees himself of the need to feed a larger number of prisoners.