That is very impressive. I've balanced on one track a few times but not in an area like that.
Glad it all worked for him, I think a bit too close to the edge for a slope cut like that especially having a slope board pulling the center of gravity more to the left. At least he's got the right machine for it, though I'd love to see that done with a modern D8. I always tried to operate smart, in a spot like that I'd want a safety line of some sort. Sure wouldn't want that cliff falling off on you, or something falling from above pushing ya over. But then being there things may look different than a video.
And thanks for this video, so far the best one I've seen doing steep work, other than the sad D155 rolling off a hill.