The 1920 LDS edition emended man here to men in order to make the number agree with the following plural pronoun they. The original text, however, has other examples of singular man, meaning ‘mankind’, taking the plural pronoun, as in the following instance that was also emended to men (in this instance, by Oliver Cowdery when he copied from 𝓞 into 𝓟):
For similar examples, see the discussion regarding every man under 2 Nephi 29:11; the original text had quite a few cases where the plural pronoun they was used to refer to generic man.
Summary: Restore in Alma 12:27 the singular man, the original reading, despite the following plural pronoun they.