I am at a loss to know why a deadstick takeoff is "not so smart? What is the risk? That the hill isn't steep enough and you trundle to the bottom below flying speed? I'd be amazed if he hadn't done a great many such hillside departures and realised how little the engine was needed - and knew from trial that it would work. That rock though...
Gravity can't fail, what's the problem? Isn't this almost the same as saying it isn't very smart to get airborne in a glider?
What is the "risk factor"? No more risk than using a 30' downslope with an engine - indeed arguably less as there is no fire risk and no distraction should the engine misbehave.
The reward? Obvious, surely? Because you can.