The 1841 British edition replaced the object pronoun form us after than with the subject form we, perhaps under the influence of the following “therefore we shall fall before them”. The original us was restored in the subsequent LDS edition (1849). Interestingly, this edition also replaced the correct we in the preceding line of text with us, giving the impossible reading “why commandest thou that us should slay these men”; there seems to have been some contamination from the copytext for the 1849 edition, a copy of the 1841 edition in which the later we (in “than we all”) had been corrected to us.
Even though the 1841 change from us to we was probably accidental, it turns out that according to the prescriptive rule we is grammatically correct since the text means ‘one of them is mightier than we are’. The grammatical issue for the Book of Mormon text is whether prescriptive grammar should be followed or not. Pronouns after than, if semantic subjects, are supposed to take subject forms (I, thou, he, she, we, and they), not object forms (me, thee, him, her, us, and them). In Old and Middle English, we would also have had the subject form ye versus the object form you, while in Early Modern English ye was eventually replaced by you. In the Book of Mormon, both ye and you serve as subject pronoun forms (see the discussion under Mosiah 4:14 and, more generally, under ye in volume 3). The problem with the prescriptive rule regarding the pronoun form after than is that speakers of modern English frequently—in fact, usually—select the object form (“Jim’s better than me”). Of course, if we expand the sentence by supplying the ellipted verb, we always use the subject form (“Jim’s better than I am”). The example here in Alma 22:20 shows the long-term preference in the text for the nonstandard us after than instead of the prescriptive we. For additional discussion regarding this prescriptive rule, see the first entry under than in Merriam Webster’s Dictionary of English Usage.
The King James Bible basically follows the prescriptive rule. Whenever a pronoun without a verb follows than and that pronoun is semantically a subject, then the subject form of the pronoun is nearly always selected (60 times), as in John 14:28: “for my Father is greater than I”. But there is one exception:
In the Book of Mormon text, there is only one case of than I:
This one instance of than I is related to John the Baptist’s language in the Synoptic Gospels (as given in the King James Bible), especially in the example from Matthew 3:11: “but he that cometh after me is mightier than I”. Much more frequent in the Book of Mormon is than they (with 11 occurrences), but there are none of than them. Here I provide a sampling:
There are two cases of than you:
As noted above, you can be interpreted as either a subject or an object form. And finally, there are two cases with object forms, the one involving us here in Alma 22:20 and one with thee:
The only case of textual variation between the subject and object pronoun forms after than is here in Alma 22:20, which is probably accidental rather than intentional. The critical text will in each case follow the earliest textual sources in choosing between the subject and object form of the pronoun after than. For further discussion, see under pronouns in volume 3.
Summary: The critical text will follow the earliest textual sources in determining whether the subject or object pronoun form should occur after than (given that the pronoun is semantically the subject); there are two cases where the object form shows up, Alma 20:17 (“than thee”) and Alma 22:20 (“than us”).