Here the 1830 typesetter changed that land to the land, perhaps accidentally. A possible source for this change is the occurrence of the land in the next clause. In the context of the verb depart, the Book of Mormon text otherwise refers to only “the land”; that is, elsewhere we have examples of “depart out of the land” (18 times), but never of “depart out of that land”. This frequency is statistically consistent with other contexts in the original text: that land is occasionally found (8 times), but the land is highly frequent (over a thousand times). In fact, for one of these other cases of that land, the 1830 typesetter made the same error of replacing that land with the land:
Note that in this second instance Oliver Cowdery himself initially wrote “to the land” in 𝓟, but then he caught his error and corrected the text to read “to that land”. Also note that in Mosiah 9:7, but not in Alma 57:29, the 1908 RLDS edition restored the that of the printer’s manuscript.
Summary: Restore in Mosiah 9:7 that land, the reading of the printer’s manuscript (“and he also commanded that his people should depart out of that land”).