However, since Bruen, they need to show that these types of restrictions are in the "historical tradition of firearm regulation". That, they cannot do.
They only need to apply Bruen if the question clears the first hurdle, which is whether or not the subject is protected under the 2A altogether. The states's claim that magazines aren't arms isn't just a twist of logic for the hell of it. It's a very deliberate attempt to avoid putting, in this case, magazine regulations to the more stringent test which Bruen dictates.
But if people read the the state's positions in the briefs, they'll see that they have part two covered as well. In other words, they aren't just banking on the idea that scotus will accept that magazines fall outside the scope of the 2A. They intend to answer Bruen methodology by pointing out "historical" precedent by way of examples that aren't directly related to magazines, or even guns, at all. In other cases, courts have accepted things like 19th century laws that restricted the sale of Bowie knives, to satisfy historical tradition of regulating arms in a broader sense.
Roberts gave the opposing side ample maneuvering room in Rahimi:
"Nevertheless, some courts have misunderstood the methodology of our recent Second Amendment cases. These precedents were not meant to suggest a law trapped in amber. As we explained in Heller, for example, the reach of the Second Amendment is not limited only to those arms that were in existence at the founding.
Rather, it “extends, prima facie, to all instruments that constitute bearable arms, even those that were not [yet] in existence.” By that same logic, the Second Amendment permits more than just those regulations identical to ones that could be found in 1791. Holding otherwise would be as mistaken as applying the protections of the right only to muskets and sabers."