I think the commentary in the handbook is on the money here. The nails or screws can't be where they would interfere with a device.
I often mount a deep 4 X 4 box against a stud...knocking out the center hole and screwing in a fender washer with a deck screw, then drilling at least two holes through the same side to install some additional deck screws. It's very secure and the screws don't interfere with the installation of the device.
I think this is a case where the legal language of the NEC makes intent as clear as mud.
Dave
