We were talking about REPLACING part of the battery with something else. Not rewireing.
The power-up test covers replacing the complete battery with some other substance (plus a little electronics), which would not show up easily on an X-Ray.
Replacing part of the battery would show up, as would replacing all the inner circuitery.
Rewiring would not show up, and would even let you add some additional electronic device, but not adding no explosives.
On a higher level, I agree with your conclusion that this whole stuff is theatre. Even Simon Beckett grade theatre, if we consider that this boils down to "Sir, would you please press the big red power button on that device of yours that we're suspecting might be a bomb?"