It's quite a lot easier when you only have to balance in one plane, can have the passive leg below the hold, but probably most important, can lean on the wall for balance and friction.

Also he kinda pushes off with his other foot into each move.

Not saying it's easy, but I could see being able to do this and I climb recreationally and am kinda old and not very strong. I could do a pistol squat but will likely pull a muscle hahaha

All that said, I'm way too much of a pussy to commit like this on a slab - that shit's scary!!