My understanding is that the ATF stopped giving determination letters to accessories, only to complete firearms, such that they couldn't get one for the bare SBA3.
with cherry picked correspondence I can see that being missed, because I can understand where ATF is coming from (besides wanting to infringe on my rights) distance between the attachment point and pistol grip differs on all firearms. Putting a brace on Mac10 and putting the same brace on Sten would be a dramatic difference in shouldering.
The thing is, ATF are not the smartest bunch, and they know it, not smarter than collective braintrust of all the a**h***s who want freedom, to make money while pushing the envelop of infringement. Collectively, there will always be someone who will come to exploit whatever loophole may be conceived and ATV simply can not cover them all.
This FOIA glimpse is good info, because frankly I thought that SB/Sig were in bed with ATF, but it shows that they are literally playing chicken and pushing their luck behind the scenes.
Unlike bumpstocks, there are a whole lot more people who like them and orders of magnitude more of them in circulation already. As much as ATF would love to confiscate the living crap out of guns, they have to tread careful and not spark the revolt that everyone masturbates to. Even if Biden sneaks into POTUS, I highly doubt that he would be worse than Obummer in gun confiscation, i.e. we'll see many small snubs like bans on importation here and there, but no door-to-door confiscations.