In practice, it means that local/state police won't/shouldn't call the ATF if they suspect someone is doing something evil with a gun, like making one with a 15

In practice, it means that local/state police won't/shouldn't call the ATF if they suspect someone is doing something evil with a gun, like making one with a 15" barrel, putting a stock on a pistol, using an oil filter as a suppressor, or 3d-printing a drop-in auto sear in order to waste money at the range more quickly. Some of the above might still be illegal under state law, of course, and those cops can arrest you for that. But if the ATF discovers that you've committed one of the above crimes (10 year felony for each), they can still roll in and arrest you/shoot your dog. They just won't have access to local support.