That's with no rider on the bike. Earlier in the video when he attempts to turn with the device that prevents countersteering, he falls off every time. It seems unlikely that 50M cyclists could easily do this but he could not.
At low speeds on e.g. a light MTB stop pedaling, lift your butt off the saddle, lean bike far into desired side balancing your body and handlebars to keep going straight. You have most of what's needed to turn. You may countersteer but your line does not veer into the direction of countersteer, counter to illustrations.
Riding edge of a cliff is not a great idea anyway because balancing a bicycle tends to require countless small adjustments even due to pedaling alone.
It's good enough if you already understand it and know what to look for; just not the best for convincing someone who doesn't believe the physics.
That's irrelevant; bicycles countersteer to keep stable, without a rider present (above a certain speed).
If the riderless experiment were repeated with a bike whose steering is locked out, it would fall down rather quick.
A steering column which somehow turns right without your hands will induce a countersteer to the left the same way as if you had used your hands.