The transitional therefore they was initially missed in the original manuscript; Oliver Cowdery later inserted these two words supralinearly with somewhat heavier ink flow, probably after he read this passage back to Joseph Smith. It is very doubtful that such an insertion would be due to conscious editing on the part of Oliver.
On the other hand, the editors for the 1920 LDS edition consciously decided to remove these two words (the change is marked in the committee copy). Of course, the pronoun they seems odd when contrasted with the singular Moroni at the beginning of this long sentence. Yet the semantic subject is the plural “Moroni with his armies”. A similar example of editing out a connective and a plural pronoun was made by Joseph Smith in his editing of the Words of Mormon 1:17–18 for the 1837 edition, namely, when he removed wherefore they from the text (see under that passage for discussion).
The connective therefore commonly occurs in the text after an initial subject and a nonfinite clause, as in this famous example at the beginning of the Book of Mormon: “I Nephi / having been born of goodly parents / therefore I was taught somewhat in all the learning of my father” (1 Nephi 1:1). The critical text will restore the original therefore they here in Alma 50:12.
Summary: Restore in Alma 50:12 the words therefore they, the corrected reading in 𝓞; although redundant, this type of expression is fully intended and can be found elsewhere in the text.