Some suggested edits like this one attempt to fix non-trivial mistakes in posts, including the addition of a sentence or paragraph explaining why. Since this does change the meaning of an answer in a severe way, we are supposed to reject such edits as too radical (we are, aren't we? but then again how does this make internet a better place?). Indeed I think this edit should have been a comment instead so the OP could either clarify a misunderstanding or learn from their mistake. As long as low-rep comments are not allowed, such an edit is unfortunately the only way (apart from a should-be-a-comment-answer) to communicate their spotting, but apart from that:
In order to make clear that such a modification should not just be an edit, should the system
- reject too long edits, since a long edit is likely to severely change the meaning, or
- at least give a warning that he suggested edit might be better suited as a comment, would you like to post one instead?