I think you are confusing Supreme Court rulings with some nonsense from Thompson Center.
Read this real slow,
Held further, a firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made when parts in a kit that were originally designed to be configured as both a pistol and a rifle are assembled or re-assembled in a configuration not regulated under the NFA (e.g., as a pistol, or a rifle with a barrel of 16 inches or more in length). Held further, a firearm, as defined by 26 U.S.C. 5845(a)(3) and (a)(4), is not made when a pistol is attached to a part or parts designed to convert the pistol into a rifle with a barrel of 16 inches or more in length, and the parts are later unassembled in a configuration not regulated under the NFA (e.g., as a pistol). Held further, a firearm, as defined by 26 U.S.C. 5845(a)(4), is made when a handgun or other weapon with an overall length of less than 26 inches, or a barrel or barrels of less than 16 inches in length, is assembled or produced from a weapon originally assembled or produced only as a rifle. Such weapons must be registered and are subject to all requirements of the NFA. To the extent this ruling may be inconsistent with any prior letter rulings, they are hereby superseded. Date approved: July 25, 2011 Kenneth E. Melson Acting Director
Then read the 4 pages that go with it from the ATF,
https://www.atf.gov/file/55526/download
Then read this whole thread, it may help your confusion as I have repeatedly stated once a rifle always a rifle. There is no pistol workaround when altering a rifle. Even if you applied for and received a SBR provision, the rifle would still be illegal. IE......there is no "SBR/Pistol" authorization.
A SBR is a specific firearm and is legal with the proper authorization. A rifle without a butt stock, no matter how long it is, is totally a no-no and plain and simply.. illegal. There is no workaround for such a firearm, no matter who is calling it a SBR, as a SBR is still a rifle, albeit a restricted one.