I am no expert but what does the treadmill experiment really prove?
If it was actually going down a real hill, the wheels would build up some resistance when transferring power to the propeller right?
But the engine of the treadmill is so powerful that it just overrides that resistance and the difference is probably what makes it go forward.
Does that make any sense?

Now if it could drive of the treadmill and just keep on going I would be impressed.