It's my understanding this is pretty typical for the ATF, in that if you possess the parts to build something in violation, they can consider that intent to construct. If I remember right they did this with suppressor "kits" or "parts" as well. Before everyone was skirting SBR rules with braced AR's this used to come up when guys would ask if they could get in trouble for just owning a SBR upper, but not having it assembled on a lower. My guess is they look at this the same way.
That said, to me it's a slipper slope (but what isn't with the ATF!) because in reality they could easily say: "Well sure you Form 1'd two SBR's with braces on them, but there's nothing stopping you from taking those braces off and putting them on the other 5 AR's you have and turning them into unregistered SBR's! It's really quite silly, I mean I own a hacksaw, so I could easily make SBR's just by sawing stocks and barrels off.
I'd also agree that like with other NFA items, if you don't do a trust you better ensure no one else has access to them.
At the end of the day, in any situation when you have one group trying to make rules to stop something, and another group trying to find loopholes, and both them have a lot of $$$ behind them, this is the type of shit show you're going to see. Doesn't matter if it's guns, Nascar, or Lance Armstrong doping for cycling.
I know paranoia and the sky is falling is a popular here, but honestly when was the last time you ever heard of someone being prosecuted by the ATF that was an otherwise quiet law abiding gun owner? In 40 years of very active shooting, I've only ever even heard of a couple first hand reports of visits from the ATF, much less any actual issue. In fact both cases revolved around someone making very large volume of gun purchases in a short time and the ATF simply interested if they were a straw seller. Neither case went past a nice friendly chat, my guess is like most investigators these guys can figure out if your a problem pretty quickly.
in 2021 the ATF recommended 16,500 cases for prosecution (That's firearms, explosives, arson, and tobacco cases), and only 7000 of those went to prosecution .......the AVERAGE for those 16,500 defendants was 8.2 prior arrests and 2.3 prior convictions. They clearly are not targeting law abiding citizens for prosecution. Buddy of mine is a retired federal prosecutor for firearms and wildlife violations, they aren't going to bother with some poor guy that has a brace sitting in the back of their gun safe unless you've done a lot of other stuff to attract their attention first.