Just adding the arm brace does not make it a legal SBR. You also need the tax stamp.It's commonly done. You take AR or AK without stock, with short barrel, and you call it a pistol. Then you add brace, and you have 'legal' short barrel semiauto rifle and you don't need SBR stamp.
I think this point is aimed specifically at this.
https://www.atf.gov/file/11816/download
The ATF is of the opinion that holding the arm brace against the shoulder constitutes a "redesign" of the firearm. The ATF says the arm brace is legal without a stamp as long as it is used as intended; as am arm brace.
The arm brace is for the moment entirely legal and I think a very effective way of improving the hold of the awkward at-15 pistol. The SBR version of the at-15 is a ton better than any at-15 pistol in my opinion.