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.
The bicycle and riding style used in the video do not provide a good illustration, I have no idea what he was trying to do as he crashed repeatedly, and he did not seem (or the frame prevented him) to use the ability of his body to lean the bike into the turn, riding naively (which didn't prevent them from talking down as if there is a clear consensus in the field of bicycle dynamics).
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.
You don't just believe the physics. If there is consensus in physics that says you cannot turn a bicycle without measurably turning the opposite way first, feel free to link. It's a ridiculous claim that assumes certain speeds, bicycle type and riding style. I've seen a guy ride a narrow rocky ridge, even jumping from stone to stone where needed (without stepping off the bicycle), was he violating laws of physics or is the world more complex than simplified models of it? Either way, it is just bikeshedding.
Now just imagine the possibilities with a rider on the bike. Being able to dynamically shift weight to either the left or right side of the bike at will! You can even do turns while riding with no hands.
If this were the case, you wouldn't be able to turn while riding with no hands. Yet reality has us out here riding bikes with no hands making turns. https://youtu.be/_SWdCutqSWQ?t=73
That is false, because you can induce a motion of the steering column without using your hands. If you shift your center of gravity around without your hands on the bars, the steering column will do things on its own and you can learn to predict and control that. Not just simply 0shifting the center of gravity in the seat; you can do things if you seize the top bar between your knees.
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.
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.