Unicyclist here. It is all leg power, but this guy is exerting himself far less than you think. There are no gears at all, geared unicycles are extremely rare and almost impossible to ride. Makes riding uphill feel barely any harder than flat land unless you're doing a slope much steeper than what's pictured.

Whenever I was using a wheel 28 inches or bigger, I beat my friends on bikes going uphill and had way more energy than them at the top. But they always beat me on the other side because I had to pedal the whole way down.

Gonna nitpick here and be pedantic. Regardless of gear ratio, it’s all leg power. Gearing simply allows the rider to go slower, which does allows for a lower required power, but for a longer duration of time. The amount of total energy output is effectively the same.

Source: cycling nerd who is obsessed with power.