>Had the test been done in a big round loop, current wouldn't start flowing at the far end immediately. Otherwise you'd have made faster than light communications.

Yes, this is obviously the case. But it's also not the setup that was being tested.

(Apparently he's editing a part2 which contains many weird & wonderful topological arrangements.)

Honestly though, I'm not sure what your problem is here. His results match very closely what you'd expect theoretically given the experimental setup. Not only that, if you model the setup in software using a suitable electromagnetic modelling code (there are videos on YouTube of people doing this) you get exactly the same result. Common mode noise on both sides is an irrelevance - you can just subtract it away without loss of generality.

You can only subtract common mode noise out if it really is common mode and perfectly equal on both sides of the system. With two discrete probes, that's going to give you a lot worse common mode rejection than with a proper differential probe. He can't even have proper ground references the way he did it (most of his probe ground clips are unconnected). It's just not a good rig. Sure, it might work, and his result might be good, I just don't feel confident about it given the experimental set-up.

If you look at his scope plot, the common mode voltage is kind of a mess and much larger than the actual differential voltage. That doesn't give me a lot of confidence in the final result. I'd like to see a more controlled version of this experiment with better equipment.

Source: I've done the same thing he did, using two probes to measure a differential voltage, and I can tell you it kind of works, but not well. Especially not at higher frequencies.

The common mode problem can be avoided by making the experiment not differential (which coincidentally saves half the wiring) but even if you do that, you still get a large influence on the result due to the coupling between the twin-lead line and ground. It's actually a pretty good demonstration why using baluns for twin-lead is necessary; only the differential mode of the line works well, because it's not actually a two-conductor line, as it's an open line it is also coupled to ground. In my experiment (with ~6 cm wire spacing, which makes relatively little difference as the impedance of the line is proportional to the log of the spacing) I found that suspending it around a meter above ground gave almost identical differential and common mode impedances. That of course rather significantly reduces the amplitude you're going to see in the experiment.

He has an upcoming part2 of the video which he listed big round loop as one of the follow-up experiments. We should just wait for that.