This comment is Mormon’s insertion in his intended text. It is short enough that it is easy to see the brackets for the repetitive resumption. In verse 6, Alma “came to a city which was called Ammonihah.” Then in verse 8, Mormon wrote: “Alma had come to the city of Ammonihah.” As with other cases of repetitive resumption (see comments on 1 Nephi 6:1–6), the material in between the repeated lines is a departure from the original.
That tells us that Mormon thought that it was somehow important to let his readers know that the name Ammonihah was related to the first ruler of the city. Unfortunately, he doesn’t tell us why it was important. Work on names in the Book of Mormon strongly indicates that many names have meanings for the text. Given the nature of the story, we might expect that this name would have an unfavorable connotation. However, the obvious root name is Ammon, and everything Mormon has said about anyone named Ammon has been good.
As a suggestion, perhaps Mormon is saying that Ammonihah began as a righteous Nephite city, but has become apostate. The name would suggest that origin, but the rest of the story will demonstrate their apostate nature.