As far as I know, there has not been ONE single case of anyone being charged for a flash hider. And yes, the correct answer is what the ATF says it is (not the manufacturer). However.... the ATF will no longer "classify" muzzle devices as it no longer regulates them (per the AWB sunset). So therefore, there are no ATF "classifications" for muzzle devices for NJ (or any ban state) to go by (therefore, Unenforceable). In other words, the ATF does not classify them. The state does not classify them (never have). So they aren't any classifications to go by for them to enforce (a manufacturer description is THEIR idea of what it is, and can advertise it however they like. It is NOT a classification).
But the rumors and worries of "getting caught" with an unlawful muzzle device keeps the fear alive (and us peasants in line)!