Sounds like it only uses human power as energy source? Storing some on flat areas / downhill / braking, and using that for accelerating & uphill.

Doubtful I'd want the added complexity + weight for that. This is what gears were invented for. Go slow(er) uphill, waste some energy on the way down.

True, smoothing out repeated uphill / downhill & start / stops is nice. But then efficiency comes looking. Is that high enough overall to make it worthwhile? (not to mention cost or reliability).

Now if supercapacitors would replace battery on regular ebike, that would be interesting.