'scuse me! How "skilled" do you need to be to roll down a hill for heaven's sake?
It's just a petrol-free way of achieving flying speed - what is so hard about a soap box derby?
It isn't a demonstration of superior skill just because it is unconventional. D'uh'oh!
Really!
Well.... Taildraggers have been known to nose over when the C of G is moved over the main wheels. The pilot would normally be able to use engine power and up elevator to hold the tail down, but this skilled pilot seems to have set up the groundroll, and then maintained it nicely until the elevator became effective, with the most adverse possible balance situation, and no advantage from engine and thrust control. Superior skill in my estimation. I think it must be a demonstration of excellent timing, or balance or something, to get the 'plane
to the point where it could be pointed down that hill power off, without nosing it over. The soapbox derby driver probably has much greater opportunity to apply brakes if needed!
Otherwise, presuming that the pilot had carefully assessed the surface as suitable and free of hazards, I see this as a low risk demonstration of skill - 'cause he can't sink and drown trapped in the 'plane, going down the hillside!