Sometimes the text uses a plural pronoun to refer to a record, as in Mosiah 12:8: “yet they shall leave a record behind them and I will preserve them for other nations”. Similarly, here in Mosiah 21:27, there is a single record that is referred to in the plural. The 1920 LDS edition changed the plural they were to it was, but the critical text will restore the original plural forms. For further discussion and a list of examples where the text uses plural pronouns to refer to a record, see under 1 Nephi 5:21.
Summary: Restore in Mosiah 21:27 the original plural they were; the Book of Mormon text often uses plural pronouns when referring to a record.