Up to a point, but as they make very clear in this proposal, Congress gave the Atty General the authority to 'interpret' the statues and create regulations to enforce it. This has been supported over and over again in court, and when it comes to a 'technical' decision like this I don't know that the ATF has ever lost a case.
They have lost at least 2 on AR-15s, which THEY chose not to appeal because they didn't want to establish a bad precedence. They didn't have a chance to get the decisions reversed.
They lost the case with T/C over whether they were selling short barreled rifles with thee Encore pistol, That one went all the way to the Supreme Court.
Those are just the 3 I can recall from memory, research might reveal more.
Granted they have some rulemaking power in the definition of firearms and what constitutes different classes of firearms. BUT it's a long way from ATF proposing a radical change in the law through regulation, and those regulations actually being implemented,