On the one hand, it's quite easy to see that the "disability equipment" angle with braces is primarily an end run around the NFA, but on the other, by allowing them to be classified as accessories rather than stocks, allowing millions of purchases to legally occur, the BATFE painted themselves into a corner regarding the idea that they're stocks.
Any change to their existing position that braces are not stocks is going to be a really hard sell, and is going to infringe upon the rights of millions of Americans. The bit where they tried to claim that a brace could be a brace on one gun and a stock on another was especially absurd. Too bad BATFE, you screwed up and now you have to live with it.