The guards escape to the Nephites. There must have been a story of how they arrived, the tale they told, and the asylum they requested. That isn’t the story Mormon was interested in. Mormon indicates only that they are sent to the people of Ammon, who themselves were refugees from the Lamanite lands. It is possible that some of the story of Amalickiah would have been transmitted by the guards, but the very fact that they went unafraid to meet Amalickiah suggests that the only story they knew of him was his last act of treachery, and not his previous ones.
The result of Amalickiah’s machinations was that he entered the city of Nephi, and became the de facto leader. He was supported by a large army against the citizens of the city who had no reason to doubt his version of the story of the death of their king.