Only in the bizarre and stranger-than-science world of the ATF. An agency that should have been terminated a long time ago!
From reading the letter it seems to be all about "intent". So as long as you don't intend to use the Sig brace (or similar) you are good to go. If you accidentally or inadvertently, but without any intent bring the brace to your shoulder, you're legal.
So, if I am holding it away from my shoulder, and the recoil causes it to brush my shoulder, do I go Federal Prison?