It's this ^. A bump stock equipped rifle does not definitionally meet the criteria of being a "machine gun" and the ATF doesn't have the authority to just say "Well, close enough." and call it good. Now that doesn't mean they couldn't go back and amend the current definition to include something like "and to include any other modification/addition, irrespective of the actual fire control mechanism, that by it's design is intended to encourage a sequence of firing functionally analogous to a "machine gun"'. This would put a bump stock under the same umbrella as an "auto sear".