Nobody knows if this is true yet. Nobody reasonable however, thinks that this would withstand scrutiny.However a barrel that is threaded is an evil feature even if it has a permently attached device.
"A muzzle cap or a PW device rendering the threads inaccessible is still illegal" as an interpretation is getting into guida glue fumes territory imo.
Not that this matters because it's never going to get tested in court regardless. (Cwofs and ple deals don't count).