You are incorrect. He is NOT going through procedures. Full auto weapons are precisely defined in law, and ATF correctly defermined that bumpfire stocks did mot comvert a firearm to full auto . By declaring bumpfire stocks to be machine guns ATF would be effectively assuming the role of congress in writing legislation. If this is allowed to stand the potential damage would be enormous, and not limited to firearms and the 2nd amendment.
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.