IMO a lightened trailer is fair. It’s really all about power to weight ratio. If a cyber truck could tow a car faster than that car can drive itself, then it means the power to weight ratio is that much higher. Torque also plays a role. They’d have to use a negative weight trailer for it’s weight to change that.
Guessing they ran the numbers and realized the instant torque of the electric motor would give them an edge at the very beginning, based on the “didn’t go the entire 1/4 mile” bit, which just means it takes less than 1/4 mile to make up for the higher torque in that particular scenario.
Also the skill of the drivers would play a role unless they set up an automated system (and tuned it right for both vehicles).