Indeed, this doesn't show which truck is more powerful / greater torque. It shows which truck is better at towing, which is arguably more relevant for a truck. Greater mass also means greater control while towing.
I don't think it's showing anything more than an entertaining bit of marketing. There are plenty of ways it could be terrible at towing but still appear better at this test.
More importantly, the wider wheelbase will be important for towing. Long, tall trailers (like RV's, or long cargo trailers) get a lot of wind force, and on smaller tow vehicles (SUV's and smaller trucks) you can get in a very dangerous situation where the tail wags the dog. For a similar reason, the heavier weight of the Tesla truck will help at stability when towing.
I think the issue with this display is the weight distribution. The Tesla is loaded down with batteries and the Ford bed is empty so it's real wheels start to spin immediately and friction of tires on road goes way down the moment that happens. I wonder if the Ford was 4WD if the front wheels would have made it more difficult for the Tesla. At any rate, it's kind of pedantic at this point, both trucks are probably good at pulling stuff, but it is interesting.
Braking is also a question of traction. Pretty much all brakes can apply a much greater force than the traction at the wheels, hence the motivation for anti-lock breaking systems. Having the truck be more massive relative to the trailer improves the breaking performance.
A more relatively massive truck also helps reduce the propensity for jackknifing.