I'm not so sure. They are leaving the door open to even consider the buffer tube a stock (#4 below). I mean they don't define the actual criteria they will use to make such determination but instead give us these "conceptual factors" that they will use. So I guess we are good if they are in a good mood that day?
They’re so vague, but they do concede that a buffer tube is necessary for the operation of an AR-15, so they can’t just flat-out declare the tube a stock. To do so would be to make every AR pistol ever made, an SBR simply because of the existence of a necessary component of the AR platform. They’d get crucified, and rightfully so. I expect this ruling to go the way of the frames ruling, only more so.