One, the Brace currently is considered not a stock - and not legal to use as a stock against the upper torso, AIW the Open Letter and it's less than precise language.
Two, we tolerate AR pistol discussions in the rifle forums. If this by legal definition isn't a shotgun (and no argument from me, I get it,) sure, it technically doesn't belong. However, since it's a "Firearm" where else do we discuss like items?
I frequent other forums where posters frequently ask questions about SBR's in the AR pistol forum. There are significant legal differences between the two, and a subforum for NFA weapons that is active. Yet the questions get posted "out of place". People tend to treat a gun by it's root origins, not it's legal classification. If it shoots shotshells then it gets posted in shotgun forums. It's also where most of the interest might lie, too, vs being an a catchall forum where they only have one consistency - being a niche gun.