Though that's not what the Supreme Court actually held when tossing the bump stock ban: they instead said that a gun equipped with a bump stock isn't a machine gun and that's why the ban was invalid.

The case entirely hinged on the definition of machinegun being a gun that fires more than one shot "by a single function of the trigger" with the majority saying that because you need to keep forward pressure to keep firing, bump stocks don't count.

The aspect of bump stocks being just a part and not a gun wasn't important to the case because the statute is clear on this point: any part that's intended to convert a regular gun to a machine gun is itself a machine gun under the relevant law.