The original manuscript reads this promise. But when Oliver Cowdery copied the text from 𝓞 into 𝓟, he accidentally copied the this as his, which is visually similar. The nonredundancy of the original reading here is preferred, although redundancies such as “his promise which he made” are found in the original text of the Book of Mormon, as in the moderately redundant “for he hath fulfilled his promise which he hath made unto our fathers” (Alma 37:17). Also see the discussion under 1 Nephi 7:17 regarding the redundancy of the original phrase “according to my faith which is in me”. The critical text will follow the earliest reading here in Alma 51:10, the nonredundant “this promise which he made”.
Summary: Restore this promise in Alma 51:10, the reading of the original manuscript.