The ATF Director's brain fart aside, the ATF has held for as long as I remember that if you had a spare stock, and an AR pistol capable of acepting that stock, you were in constructive possesion of an SBR. That's the whole reason smooth "pistol" buffer tubes exist, so you could have a tube that didn't accept any stock lying around.
There was never any reason to think once they decided a pistol brace was a stock that they changed their mind on the constructiv posssession thing.
The rule itself says your options are:
1. Remove the short barrel and attach a 16-inch or longer
rifled barrel to the firearm.
2. Permanently remove and dispose of, or alter, the
“stabilizing brace” such that it cannot be reattached.
3. Turn the firearm into your local ATF office.
4. Destroy the firearm.
5. Register the weapon as set forth below depending on
your category of possessor
bolding mine. Basically the Directorwas mistaken and/or full of *expletive deleted*it and the ATF is making sure everyone knows what he sadi isn't correct.
ETA and Side Note: Many pistol braces slide on to a normal carbine 4 or 6 position reciever extension. While removing and destroying/altering the brace would solve the pistol brace issue arrising from this rule (if that's your choice) you will then be left withan AR pistol sporting a bare Reciever Extension. If you have any other stocks lying around (or possibly on other guns, it's unclear) you'll be in old school 90's style constructive posssesion of an SBR. If that's you choice of compliance, smart money would alter the pistol so it won't except stocks, either by swapping to a smooth "pistol" RE, or modifying your carbine RE so it won't accept a stock.