Have you ever heard of bending an Upper receiver?

[Ashe]

The threads are on the Upper Receiver, the barrel nut is torqued to the threads, the barrel nut is not attached to the barrel or the barrel extension. So applying torque to the barrel nut with the upper properly viced will not do anything to the barrel, However applying torque to the barrel nut with the barrel clamped (No matter how you do it) will still apply all of that torque to the locating pin on the barrel extension.
Same as if you clamp the upper receiver and apply torque to the muzzle device.