If registration is disabled and a new user attempts to register by login with Facebook, XenForo try to associate an existing account instead of displaying the message "New registrations are currently not being accepted".
That was actually intentional, but I've changed the code a little. If you would have to associate an existing account, it will let you. Otherwise, you'll get the registration disabled error. To be required to associate, you'd either be logged in or have a FB email that matches one on the board.
Note that logging in and registering with FB happen through the same system and there's no way to distinguish between the behaviors until we get info back from FB.