Oliver Cowdery’s correction in the text at the end of this passage (from it is to they are) is very probably the result of conscious editing. The singular it is in the original reading for 𝓟 jarringly conflicts with the preceding plural his garments. Earlier in this passage, scribe 2 of 𝓟 initially wrote “except his garment is cleansed from all stain through”, undoubtedly because as he copied from 𝓞 into 𝓟, his eye skipped down one line of the original manuscript to read “it is cleansed from all stain through the blood of him”. Scribe 2 caught his copy error, correcting garment to garments, crossing out “is cleansed from all stain through”, and then writing inline the correct “are washed white”.
The original Book of Mormon text sometimes uses a singular pronoun to refer to a plural noun that acts as a unit, such as it to refer to these records in Mosiah 28:17: “now after Mosiah had finished translating these records / behold it gave an account of the people which was destroyed”. Of course, most of the time, the pronoun used to refer to a plural records is the plural they or them. Here in Alma 5:21, the it is the only example of a singular pronoun referring to garments; elsewhere the text uses the plural they or them:
The critical text will retain the fully intended but difficult reading in Alma 5:21: “yea his garments must be purified until it is cleansed from all stain”.
Summary: Restore the original singular it is in Alma 5:21 since scribe 2’s initial error “his garment is cleansed from all stain through” shows that the following line in the original manuscript actually read “it is cleansed from all stain through”; the original text sometimes uses the singular pronoun it to refer to plural nouns that act as a unit.