Here we learn something new that we did not know when we first saw this story in Mosiah 7; Ammon and his brethren are originally mistaken for the priests of Noah. In chapter 7 we are allowed the misperception that they would have thought them to be Lamanites. However, with the information now available, that could not have been the case, as the Limhites were tributaries to the Lamanites and the capture of Lamanites would be an act of war, an event that Limhi would want to avoid. Thus it is the suspicion that these are the priests of Noah that leads to the capture and imprisonment of Ammon, Helem, and Hem (see Mosiah 7:6).
This information suggests that Limhi's guard were younger men who were not part of Noah's court, else they should have recognized the priests of Noah. They could tell that Ammon, Helem, and Hem were not Lamanites, but they clearly did not recognize them either as the priests whom they could have been, or the Nephites, whom they were.
Ammon spent two days in prison (Mosiah 7:8) and certainly in this time there was a determination made that they were not the priests of Noah. Since they were not the priests of Noah, and were certainly not Lamanites, Ammon is given the audience before Limhi to discover just who they were (Mosiah 7:9-13).