Technically I'm measuring in the "wrong direction" since I would be very surprised if an EMP were initiated inside a trash can in my basement, but shielding works the same in both directions so this doesn't matter.
It really doesn't, with 1.5GHz you're still in near field regime inside the can.
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.
Yep. The exact placement of the monopole w/r to the close can wall will change the the polarization which changes how will it will get through the imperfections in the horizontal seam slots which will be polarized. And that's ignoring the that different places will be better or worse (vs frequency) for exciting the can's cavity modes (at $frequency) in the first place.
But a long distance high power RF source can have these properties altered too by the type of emission mechanisn, reflections, scattering, etc and items placed in such a faraday cage will be located at good and bad places to excite the cavity mode too.
So it's kind of a wash and probably okay for this kind of test. The fact that the electric and magnetic components of the field aren't in balance (near field) doesn't matter too much.