The original text of the Book of Mormon has numerous examples of the pronoun them used as a determiner, as in “all them whom he hath chosen”. In most instances, Joseph Smith edited this dialectal use of them to the standard English determiner those. See the complete discussion under pronominal determiners in volume 3.
Summary: The original text has many examples of the pronoun them used as a determiner; these examples have usually been edited to those, as here in 1 Nephi 1:20; based on the earliest textual sources, we restore the original reading (“all them whom he hath chosen”).