Here in the printer’s manuscript, Joseph Smith changed they to those when he marked up 𝓟 in his editing for the 1837 edition. But this change was never implemented in that edition (or any other one), probably because it was realized that “these are they” is a normal expression in the Book of Mormon. There are 13 instances of “these are they” in the original text, none of which have been edited to “these are those” (except here in 𝓟 for Alma 5:56). On the other hand, there are no occurrences in the original text of “these are those”—and only one in the current text, in Mosiah 15:24, which originally read “there are those” (for discussion, see under that passage). For further discussion of the general editing of they to those, see under pronominal determiners in volume 3. Here in Alma 5:56, Joseph also made his typical grammatical change of which to who (since the relative pronoun refers to people). For a complete discussion of that change, see under which in volume 3.
Summary: Retain the original and more expected “these are they” in Alma 5:56.