Ok, so it looks like at least for now, all we know is that the ATF has applied this to offerings from Q specifically (the Sugar Weasel and Mini Fix), and it sounds like this ruling is at least in part due to how the ATF has determined that Q describes these products on their website, suggesting that they are designed to be fired from the shoulder, and thus are considered SBRs.
It will be interesting to see where this goes, and if/how it ends up getting extrapolated to all pistol braces, but that seems far from conclusive at this time. It also seems like the ATF is taking issue specifically with how Q is describing the use of these products, and thus may come down to how a company describes the usage of the products they sell with pistol braces, rather than the pistol brace itself.
Of course, this is super murky territory, and shouldn't be a much of a surprise. Those of us paying attention to the legal status and approved usage of pistol braces for some time have been wondering when this might happen.