The original manuscript is not extant here for the word thus. The printer’s manuscript reads thus, but there is a definite i-dot over the u of thus, which means that this word could also be read as this. The 1830 printer set thus, which is in accord with the rest of Book of Mormon usage: elsewhere there are 41 instances in the original text of “thus ended the Xth year” but none of “this ended the Xth year”. There are two instances in the text of this end, where end is a noun; but there are no instances where this is followed by a form of the verb end. This implies that this ended would be highly unexpected, although one could interpret “this ended the Xth year” as meaning ‘this event ended the Xth year’.
The manuscripts show numerous places where this and thus have been mixed up (see under Alma 11:21 for a list). In two other places, Oliver Cowdery initially wrote this in place of thus in the phrase “thus ended the Xth year”:
Here in Alma 63:3, it is quite clear that the extra i- dot for thus is a scribal slip.
Summary: In Alma 63:3 the printer’s manuscript could be read as either thus ended or this ended, but the i-dot over the u of thus is undoubtedly an error; elsewhere in the text this is not followed by the verb end, while “thus ended the Xth year” is very common.