Here the 1830 compositor replaced this land with the land, perhaps because this already occurred twice in this sentence (“throughout all this land among all this people”). Most likely, the replacement of this with the was simply an accident. The 1908 RLDS edition restored the original this land to the RLDS text. There is no reason why the more specific reading with this, the earliest reading, should be rejected. For discussion of two other cases where an original this land was accidentally replaced by the land, see 2 Nephi 1:8 and Helaman 14:20.
Summary: Restore in Mosiah 1:10 this land, the reading of the printer’s manuscript (the earliest source here).