While most of the time a suggested edit on a post is reviewed by users with the editing privilege, the author of a post can always review an edit suggested by somebody else on their post, too. Indeed, the vote of an OP is stronger than a normal vote, it alone can decide on the edit taking effect or being rejected (normally it takes two users).
For this and additional information see How do suggested edits work? (especially the section "Who can vote on a suggested edit?" in the answer).
Thus, it can happen that a new user reviews an edit. Yet, only if it is an edit proposed by somebody on their own post; this is what happened in the current case https://mathoverflow.net/review/suggested-edits/23344
