“That is they had fallen into the hands of the Amalekites and the Amulonites (see Alma 21:1–4). We know that Amulon was one of the wicked priests of Noah (Mosiah 23:32). The present text is silent as to the identity of Amaleki” (McConkie and Millet, Doctrinal Commentary, 3:149–50).
The superscription to Alma 21 was part of the ancient record translated by the Prophet Joseph Smith and dictated by him to his scribe (see also, for example, 2 Nephi, Mosiah 9, and Alma 36).
The italicized words “Comprising chapters 21 through 25” were not part of the ancient record but were added later to printed editions of the Book of Mormon.