In slow-flight configuration, I could get the 150 to be flying a few feet above the ground at about 30 mph with no wind. All it would take would be a 40 mph wind to go backwards
Loved the "physics" of slow flight.
That thought about slow flight physics just might explain the short take off.
In regular flight attitude, speed is controlled by the throttle and altitude by the attack angle...point the nose up, the plane goes up, point it down and it goes down.
In slow flight, you have full flaps on, which that Cub did, and you have a nose high attitude which make the plane like an air brake...lots of drag.
In that attitude, you increase speed by lowering the nose a bit, reducing drag and increasing speed. You gain altitude by giving it throttle and the prop will pull the plane up a bit....everything is happening by only a very small amount and very slowly.
That Cub, sitting on the hill with full flaps and brakes was in slow flight attitude sitting still. To take off, you could hear him gun the engine and then release the breaks. The plane, being in that attitude, responded by gaining a few inches of altitude. If the wind was near 30mph, maybe less for a Cub than the 150, that could be how it was don.