Editing the tags by itself is already enough to make an edit qualify for review. Refusing <6 character edits to the body on a tag edit makes no sense.
The whole point of blocking too minor edits is to prevent using up reviewing resources on edits which are not important. Because a question's title and tags are important, an edit to either already clears the bar for being "review worthy". The system allows such edits through without question.
For this very reason, <6 character edits to the body of a post are allowed when the title is edited; see here. In a lot of cases, a question's tags can be as important as the title, if not more so. The same should apply in the case of tags.
allowing encouraging editors to fix everything in a post is a much better idea than to suggest that tag edits and body edits should be done separately. Furthermore, enforcing the character limit when a user is editing a post already will encourage unnecessary changes to the body, just to get around it.
Finally, implementing this would fix this bug, in which tag-only edits to certain specific posts are disallowed due to the system seeing small body edits that weren't actually made by the editor.