I can definitely see how he only makes it look easy. I bet I'd fall on my ass on the rolling pin, incredible. If I had to guess, I'd say that the stabilizing core muscles are being activated in ways that normally wouldn't be on an even surface, and on top of being strengthened, I'd guess they're also being conditioned to endure, since the positions are being held continuously. Is that right?
Running is the easiest sport to get into which appeals to Kenyans who generally live in impoverished conditions. Also long distance runners are usually thin people so they would be at a very low weight class like bantamweight or even lower and theres no money in those divisions so theres even more obstacles there. Kenyans are also motivated to run because of the greatness of their country in the sport. I do think Kenyans would dominate boxing if they were in the right circumstances.
On the Rollers He is Biomechanicaly moving in the opposite direction to where He is going. On the Skate board Roller, He is working on the ability to Kick with either Foot. Everything else is as you say. Heres a clip from 3 years ago, gives you an idea . https://www.youtube.com/watch?v=ieOAiQVnE6Q
Did you know that there is no transfer from exercises on wobble boards, rollers etc. to flat ground exercise?