Here we may have an instance of mishearing them as him since both are colloquially pronounced as /ßm/. As explained under 1 Nephi 10:18–19, there is evidence in 𝓞 that these two pronoun forms were occasionally mixed up. Here in Moroni 8:16, the 1920 LDS edition emended the earlier him to them so that there would be agreement with the two instances of plural they later on in the sentence (“for they shall perish except they repent”). Even so, there is clear evidence in the original text that generic pronouns can occur in either the singular or the plural and that switches in grammatical number for these pronouns can occur within the same sentence. For some examples involving whoso(ever), see under 1 Nephi 17:48. The critical text will therefore allow for such switches in generic pronouns, when supported by the earliest textual sources. Thus here in Moroni 8:16, the earliest reading will be restored, with him near the beginning but switching to they later on.
Summary: Restore the singular pronoun form him in Moroni 8:16, the reading of the earliest text (the printer’s manuscript); although him could be an error for them, switches from singular to plural in generic pronouns can be found elsewhere in the original text.