Great question.
That is definitely one of the big caveats of BEVs over diesels. A battery on an EV can only take in so much energy. Once you hit that ceiling, the battery won't take in any more current. Fun fact, having a super charged battery in a BEV causes all sorts of headache and can cost you performance.
You either have to switch back to service brakes or, as you mentioned, burn off energy as heat. Not sure how they're doing it with this truck, but on other BEV loaders which I've worked on, we add a hydraulic valve whose only purpose is to create flow, pressure, and subsequently heat. It basically just adds a dummy load. I suspect they tapped into the dump hydraulics and added such a valve for this truck.
Sheeiit I absolutely loved the first one, even only having played it for the first time last year. I was blown away at how well it holds up. I do recommend trying it if you have even a morsel of interest. Just my personal 2 cents tho.