Actually, arm braces are not absolutely defined as not stocks by the ATF. In it's 2017 letter, the ATF specifically states that if the brace is altered or used as a stock, it's a stock.
If they use it solely in its original design and for primarily an arm brace it is still a brace. So it depends on how it is used and installed. If it's length lends it to be used primarily as a stock, it's a stock. I copied and pasted the relevant part of the letter.
With respect to stabilizing braces, ATF has concluded that attaching the brace to a handgun as a forearm brace does not "make" a short-barreled rifle because in the configuration as submitted to and approved by FATD, it is not intended to be and cannot comfortably be fired from the shoulder. If, however, the shooter/possessor takes affirmative steps to configure the device for use as a shoulder-stock -- for example, configuring the brace so as to permanently affix it to the end of a buffer tube, (thereby creating a length that has no other purpose than to facilitate its use as a stock), removing the arm-strap, or otherwise undermining its ability to be used as a brace - and then in fact shoots the firearm from the shoulder using the accessory as a shoulder stock, that person has objectively "redesigned" the firearm for purposes of the NFA.