So about a month ago the ATF said the Honey Badger pistol from Q is an SBR. They never gave a reason just that they believe it is.
Now as for with pistol braces currently its legal to shoulder them and have them on a pistol and all that, but the ATF may decide to change that in the future as they have not made a final ruling on them. What's been happening basically ever since the Bumpstock ban is the ATF has been given the go ahead to declare/redefine whatever they want as x thing instead, which has lead to several firearms being deemed as SBSs, some shotguns getting banned because the name is used by a full auto version so therefore its a full auto, changing the definition of a firearm, the Bumpstock being declared as a machine gun, etc.