That device in the opening post looks to me like a simple grip stop, but I am not an enforcement agency. I personally can see uses for that device as a hand stop for a horizontal grip:
o For a repeatable grip position for sake of accuracy, I made a hand stop by wrapping a stock with loops of parachute cord.
o In the same DIY spirit, with a short but legal long gun I attached a loop of web strapping to the forearm so my hand could not slip in front of the muzzle.
I see the device as being useful as a grip stop to enhance safety and accuracy. But, as a vertical grip accomodating maybe one finger on an AR pistol, that device would be useless for that purpose, in my layman opinion.
The part of me inherited from the coal mining country side of the family wants to scoff at bull manure laws, but I do advise checking with ATF Firearms Technology Branch and getting a letter. But watch carefully the wording of your question: stay clear and on point.
Side track: the SIG brace I looked at was awfully short and useless as a buttstock to me. It also did seem clumsy to me as an armbrace. Since I am two handed, I saw no need; but, if I had an AR pistol, it could work as a counterweight to balance the gun at the handgrip one handed. And I might want it for the same reason my wife bought a copy of Salman Rushdie's "Satanic Verses" -- just because they said no.