It's not a shotgun. A shotgun is defined as meant to be fired from the shoulder. No stock (ever) = not a shotgun.
This is the exact same thing as any other pistol grip only shot firing firearm. Over 26 inches so not an AOW, no stock (ever) so not a SBS. 99% of the laws, while arbitrary, aren't very capricious... they are usually black and white with very little gray. Is it A or B? Does it have this or that? There is no, "but what if...?" does it have A or does it have B? The SIG brace and the Shockwave Blade are notable exceptions.