If it can be made to function as a firearm sound suppressor (and that one will), then it's a firearm sound suppressor, and subject to the NFA in the USA. That the thread is not a standard firearm barrel thread doesn't matter.
Hell, ATF has declared some airsoft receivers to be machineguns because they could be adapted to work with an AR-15 upper assembly and discharge a shot, though that's certainly not what they're designed, intended to be, or generally used for.

