The original manuscript is extant here and reads that land. The reference is to “the land of Zarahemla”, found near the end of the previous verse: “we did inquire of Gid concerning the prisoners which they had started to go down to the land of Zarahemla with” (verse 28). When Oliver Cowdery copied verse 29 into the printer’s manuscript, he initially wrote “the land of Zarahemla”, probably because of the preceding “the land of Zarahemla”. He may have also been influenced by the use of the same full phrase in the next line of 𝓞, in verse 30: “we did start to go down to the land of Zarahemla”. For verse 29 Oliver caught his error in 𝓟: he overwrote the e of the with an a and inserted a t inline. He also crossed out of Zarahemla. However, the 1830 compositor set the incorrect the instead of that, thus giving the current text: “which was appointed to guard them down to the land”. The phrase the land definitely sounds incomplete. The critical text will, of course, restore the original that land, which is extant in 𝓞.
Summary: In accord with the reading of 𝓞 and the corrected reading in 𝓟, restore the determiner that in Alma 57:29: “to guard them down to that land”.