Regardless, even with a small gradient, the kinetic energy can never need 100% converted back into stored energy into the battery. There will always be a loss.
He definitely burns through the battery much quicker in mountains, and going down it recoups just a small fraction of energy spent going up. Also cold messes up the numbers pretty significantly, I think he mentioned 10-15% of battery consumed standing still each full day in freezing temperatures (he told me battery keeps itself warm).
Pushing up 1,5 tonnes (or 2+ in tesla's case) of our ICE car can burn up any fuel very fast in relatively short distance. I've estimated that our current petrol and previous (otherwise very efficient) diesel burn around 1/4 to 1/3 of the fuel tank on each 1000m of altitude gained.
Cruising along on a level highway at around 60 MPH, my EV uses like 2-3% the available power. Going up a incline and you're suddenly way above 40%.
This should come as no surprise to those who have ridden a bicycle outside of Holland.
(No offense intended.)