Tried this Plugin on a user A and B in mediawiki 1.37.4
It said that the users A was merged successfully to B, but in the history of a random-picked sample page the edits still were listed as user A.
I repeated the drill and selected "Delete old user". After that the edit-logs of user "A" were all gone. The edit history had only records for other users.
My expectation would be, that edits from user "A" are beeing listed as from user "B" after the merge.