A moderator can edit a post's revision comment (a.k.a. edit summary). However, when the text is too short, they get an error message that says:
Your edit summary must be at least 10 characters.
On the other hand, users with the "edit questions and answers" privilege can put a 1-letter edit summary if they want (not really a good idea, but to show the comparison...).
AFAIK, this tool has rare use cases, so I don't think removing the minimum length makes it more abusable.