So I learned that Users.Id == -1 refers to a group of anonymous users
This is where you're getting tripped up, because it's just not true. The Community user simply takes ownership over certain actions that should have a user attached, but don't always. In the case of suggested edits, the Community user performs the final edit once the suggestion is approved, even though the author of the initial suggestion is anonymous.
In the database, -1 always refers to something attached to the Community user - something the system took responsibility for due to something else happening in the system. That something else isn't always related to an anonymous action, such as in the case of casting the final Approve or Reject vote when another user Improves an edit, or when the author of a question agrees with a duplicate close vote. Those weren't anonymous actions; we just don't attribute them to the user who was actually kind of responsible for causing them.
Anonymous users can't actually do anything other than suggest edits on our sites. So that's pretty much the only place you'll ever find a "truly anonymous" user that never existed. However, the database does not differentiate between different types of anonymous users. They're all just a null user ID in the database with some arbitrary text assigned as a display name, and could occur due to a variety of reasons:
- An unauthenticated user suggested an edit.
- The user was deleted.
- The post was disassociated from the author's account.
- The post was migrated and the user didn't have an account on the new site.