In the original manuscript, Oliver Cowdery initially wrote man, then immediately corrected the a to an e by overwriting the a with an e (there is no change in the level of ink flow). But then to make the word perfectly clear, Oliver crossed out his corrected men and rewrote the plural men inline.
The use of the plural men here is supported by occurrences of the plural men in surrounding clauses:
The corrected men in 𝓞 is undoubtedly the original reading for the phrase “the great call of the diligence of men”. For a list of cases involving the mix-up of man and men, see under 1 Nephi 15:35.
Summary: Accept the corrected men in Alma 28:14 since the correction of man to men was immediate; both preceding and following clauses also have the plural men.