The edge cases aren't really the authors who claim something is fanfic or not and might be lying - the edge cases are the fannish authors who would like to archive on AO3, but truly don't know if their own work is or isn't, and don't want to have to worry about whether they're violating the rules. And it can make people feel uncomfortable in fandom if the feel like they do have to worry about that every time they post.

A lot of people (many of them fanauthors) don't think authorial intent is a valid way to judge a work's category. So when you say "just let the author decide!" my answer is "I can't!", and thus I find that solution unhelpful.