The 1830 compositor seems to have accidentally dropped the determiner this from this passage. There are eight occurrences of the phrase “this my people” elsewhere in the text, with five more in Mosiah (1:10, 10:18, 12:1, 12:4, and 29:32) and three in 3 Nephi (3:6, 3:10, and 21:2). The 1830 compositor correctly set each of these other instances of “this my people”, so there is no reason to consider the 1830 compositor’s omission here of the this as some kind of grammatical or stylistic emendation to the text. In accord with the printer’s manuscript, the 1908 RLDS edition restored the this to the RLDS text.
There are a number of occurrences of the noun phrase “this my X” in the King James Bible, including “this my oath” (Genesis 24:8), “this my lord” (Daniel 10:17), “this my son” (Luke 15:24), “this my joy” (John 3:29), and “this my tabernacle” (2 Peter 1:14). Besides the nine original occurrences of “this my people” in the Book of Mormon, there is also one occurrence of “this my joy” (in Mormon 2:13).
Summary: Restore the original reading “this my people” in Mosiah 24:13; this phrase is fairly common in the Book of Mormon text.