Ammoron appears to consider himself is a very strong position. He is suggesting that he exchange his captured prisoners not for Nephite prisoners, but for a city. In some ways, this would seem to be a greater prize than an exchange for people, but the events that Helaman just described had destroyed a large part of the garrison defending Antiparah. Ammoron certainly knew that this once strongly held city was now much weaker, and likely to fall. He was willing to sacrifice it to have the return of fighting men.
Even with Antiparah taken, he still held Cumeni, Zeezrom, and Manti. It would appear that he was content to focus his thrust into the Sidon valley through the Manti entrance. This gave him both Cumeni and Zeezrom as buffer cities on the rear of his attacking force. Ammoron was really giving up nothing, but hoping to gain military strength for his push into the Sidon valley without sacrificing anything that was of military importance.
Helaman does not take the bait. Helaman apparently knows that they have severely weakened Antiparah, and he supposes that the Nephites may retake it without great effort. He sees the exchange as valueless for the Nephites, though he may not have understood the nature of the coming Lamanite offensive plan.
Textual: This is a continuation of the letter of Helaman to Moroni. There was not chapter break in Mormon’s text, and the break here is entirely artificial. The letter is being written in the thirtieth year of the reign of the judges. At this point in Helaman’s recitation of past events, he is approaching the end of the twenty eighth year.