What you say is true, and that is exactly why he is going through procedures. What those procedures actually involve, I have no clue, but he is indeed not just making law from his pen. In fact, you can still legally buy a bump stock.
You’re missing the poimt. Trump directed DOJ and ATF to ban bumpfire stocks. When ATF implements a regulation they first issue a Notice of Proposed Rulemaking, which states what the new regulation will be and when it will become effective. The Notice also includes a comment period, when the serfs are allowed to talk back to their betters. In rare cases ATF has actually changed the proposed rule due to feedback. The issue here is that ATF has been directed to change the legislated definition of a machine gun through regulatory action. Constitutionally, this MUST be done by congress.