I find it fascinating that it works at this scale, but the basic math makes sense. The principle is that the rocks are closer to the top of the mountain, so if the dropoff point is lower than the pickup:
Potential Energy used going downhill = (Rocks + Truck) × gravity × height
Potential energy returned going uphill = (Truck) × gravity × height.
So as long as the conversion, regen braking efficiency losses, friction and other losses from driving the truck don’t exceed rocks × gravity × height, it’s a net energy gain for the battery.
Even if it’s not a net gain, it’s way less than pure Diesel and they can top up on breaks hopefully. It’s a similar principle to Edison’s proposed hybrid trucks.
Which is not new, self generating electric mining trucks have been a thing for years.
Also a thing with logging trucks that go up the mountain empty, but down the mountain with 140,000 lbs of log.