A 22 mm NATO flash suppressor constitutes a "grenade launcher" when it's mounted on a firearm in terms of defining whether or not the firearm is a federal "semiautomatic assault weapon", but it is not a grenade launcher in the sense that an M203 40mm launcher is a grenade launcher (i.e. a destructive device). A grenade launching accessory for a rifle can't fire fixed ammunition.
The passive grenade-launcherness (sorry about that) of a NATO flash suppressor is trumped by the fact that a FAB-10 does not accept detachable magazines. Under federal law you can configure one any way you like as long as it meets the minimum barrel length and overall length requirements.
The key to staying legal is not to use a rifle grenade launcher to launch a live grenade, i.e. a destructive device, without paying the tax, doing the paperwork, and complying with state and local laws.
I have a grenade launching device that screws onto the end of a Swedish Mauser. All by itself, or even mounted on a bolt-action rifle, it's legal even in Californistan.