That was because of the technical definition of a short barreled shotgun which isn't the same as a handgun. A handgun with a smooth bore that is not a muzzle-loader is a SBS under the NFA. I don't think ATFE will reverse this decision, because it addressed the main problem - the item in question is perfectly legal, how it's used is up to the user. When they reversed themselves on things, like bumpfire stocks, it wasn't because of how the device was used, but how it operated. So, no, I think the SiG brace is here to stay, but there will be opportunists who will push the envelope as hard as they can to make a quick buck - remember when the firearms industry begged Olympic Arms not to make a 7.62x39mm pistol? oly said "up yours", and put out an Ar based 7.62x39mm pistol - the next day all steel core import ammo was classified as armor piercing because there was now a pistol that could fire it, and banned from import.
