Apparently it's all about intent if you intended to make an SBR and instead bought a brace and used it as such then you are breaking the law. If you bought a pistol then happened to shoulder your pistol that is not illegal. Not advocating to try this out not worth messing with the ATF
^This. The key word is "intent".
“[a]ny person who intends to use a handgun stabilizing brace as a shoulder stock on a pistol (having a rifled barrel under 16 inches in length or a smooth bore firearm with a barrel under 18 inches in length) must first file an ATF Form 1 and pay the applicable tax because the resulting firearm will be subject to all provisions of the NFA.”
The reason they released the statement is because they kept getting letters from people (it boiled down to one specific person when they sent out the release) asking if it was OK to build a pistol with the Sig brace and shoulder it. If you build a pistol with the intent to shoulder it, then you are not building a pistol per the ATF definition. As long as the intent is to build a pistol with the intent to use it as a pistol, then you are legal. If you happen to shoulder it later, then that's not the issue. The issue is the builder's intent when building the weapon. If you don't tell anyone anything about your plans, then it can be assumed your intent is a legal pistol with a Sig brace attached.