no improvement whatsoever
This edit does not make the post even a little bit easier to read, easier to find, more accurate or more accessible. Changes are either completely superfluous or actively harm readability.
When I first got this message after an edit of mine was rejected, I already found it quite a harsh remark from the reviewer. Then I found out it is generated automatically. Now, when I'm reviewing myself, I hesitate using this reject edit reason, because of its wording. Other reject reasons are more nuanced.
So I'd like to propose a slightly different wording:
little or no improvement
This edit does not make the post sufficiently easier to read, easier to find, more accurate or more accessible. Changes are either trivial or superfluous, or do not contribute to better readability.