Here in his editing for the 1837 edition, Joseph Smith emended themselves to his people since there was no specific referent for the reflexive pronoun themselves at the beginning of this chapter. (There is also a new chapter here in the original chapter system, namely chapter XXII.) Of course, the context implies that themselves refers to the Nephites. The last sentence in the previous chapter ends with a reference to the people of Nephi in its description of those who had been “sent forth to preach among the people” (Alma 49:30). Thus there is nothing especially difficult about determining the referent for the reflexive pronoun themselves at the beginning of the new chapter. It should also be pointed out that the reflexive form makes sure that Moroni himself is included as part of the referent (in distinction to the emendation his people). The critical text will restore the original reading here; it is not a mistake, nor does it cause any real difficulty in understanding.
Summary: Restore in Alma 50:1 the original reflexive pronoun themselves; even though there is no explicit antecedent for themselves at the beginning of chapter 50, the pronoun is easily determined as referring to both Moroni and his people.