In this passage, the original text distinguishes between Moroni’s men who were with him and all the men of his army. The 1830 edition replaced the his with the in an attempt, perhaps unintentional, to avoid the redundancy with the following relative clause (“which were with him”). Elsewhere in the text, there are a number of places where such redundancies have been left unedited:
Under 1 Nephi 7:17, I discuss the redundancy of the original text for that passage, namely, “according to my faith which is in me”. The critical text will accept all examples of this kind of redundancy, provided there is support for the reading in the earliest textual sources. Thus his will be restored here in Alma 55:21 (“his men which were with him”).
Summary: Restore in Alma 55:21 the original possessive pronoun his, despite the redundancy of the following relative clause (thus, “his men which were with him”); this kind of redundancy can be found elsewhere in the text.