He’s testing penetration from the opposite side regardless of the source location.

Yeah, you put a transmitter inside the can, and a receiver inside the can, and the receiver is gonna get signal.

As GP pointed out, the problem is that if the source is inside the can, the walls are near the source - where the EM field diverges - whereas with an EMP the source would be far away (hopefully) so the EM field lines would be roughly parallel over the surface of the cage