While he can't do that, what he actually can do is slow-walk or outright pause practically everything. A great example is when Texas passed the Texas Suppressor Freedom Bill, which states that suppressors designed, manufactured, and sold in Texas only to Texas citizens made with Texas materials must be exempt from the Interstate Commerce Clause, and thus outside of the ATFs purview. The ATF immediately sued of course, and last update was that a Federal judge tossed the case as there was no injured party yet.

A based ATF director could order his agency to simply concede all suits brought against the NFA, GCA, etc. ATF lawyers walk into the court and on day 1 just tell the judge "we concede the case to Texas (or whoever) and ask that you rule in their favor. We will offer no testimony, evidence, or documentation against any of the plaintiff's claims".

Open season on any provision of federal gun control for any state AG to make a case, with the assumption that the feds won't even try to argue in their own favor.