That sad part is DIY sites tell HO to take a tester to the box. If it lights, you may now proceed to install 3 prong outlets... just bond to the back of the box.JMHO, but the jacket is 'wound' and (i'm told) 5-6 X's the length of interior conductors
they found this out with the old BX
if the shell was energized , it wouldn't trip
but not always because it was made off K&T, because of VD
we'd see this line of dots, in the old horsehair plaster...and we knew....
Home Center books are the same, creating the idea armor is always an low impedance path: