You need to read through the specific pages that deal with what is and what isn't considered an SBR.
From my reading (a few times) it isn't crystal clear but looks like a naked buffer tube is just one of many factors in how they will judge it as being an SBR or not.
There are still many gray areas as it is the Government but it seems based on rear surface area (non defined) in conjunction with other non-pistol related accessories.
So (just my take) it looks like an under 16" barrel, bare buffer tube (Non-extended, and/or not spaced back, no locking notches or stock position locking holes), that is about 6"- 6-1/2" long, on a pistol with conventional sights, or a red dot, with unlimited eye relief will still be a pistol.
On the other hand-- same gun--- under 16" barrel, with bare buffer tube (Non-extended and/or not spaced back), that is 6"- 6-1/2" long, on a pistol with a scope that has as short limited eye relief (basically needs to be shouldered for scope eye relief to work) would (or could) be then considered an SBR.
There is also some language that points to how the gun compares to a like gun that has an over 16" barrel and conventional stock, like total weight, trigger length-of-pull, etc.
So (again my take) if you have a bare bones under 16" barreled AR pistol, with open sights, a standard (round-non-locking-notch buffer tube) that is a standard carbine length buffer tube, that has a trigger length-of-pull that is well below the standard 13.5"-14.5" rifle length-of-pull then that will be considered a pistol.
From there (again my take) the more you add in rear surface area, add in short eye relief accessories, increase the trigger length-of-pull, or add weight, then the more chance it could be ruled a SBR.