If the barrel length is shorter than 16" or if the overall length is shorter than 26", and the gun has a stock, then its an SBR. This requires registration with ATF and a $200 tax stamp.
If it has no stock, then regardless of barrel length or overall length its a pistol. Whether it has a 'pistol brace' or not does not matter.
In simplest terms:
The brace can be used as a stock, but because it's 'intended use' is to support single-handed shooting, it can circumvent this law.
Brace in its intended use-case:
Brace used as a stock: