A Doubting Thomas here. Drag. The drag produced by the platform is drag, plus the drag of the blades, plus the most important fact- the energy required by the moving vehicle to rotate the generator is largest factor.
Even our modern day alternators rob the engine of horsepower ( energy), I would be surprised if all the drag and strain on the engine ( more gasoline required) would barely exceed the “ windmill” energy output.
https://thepowerfacts.com/how-much-power-is-needed-to-turn-an-alternator/
I think he’s using the car to test his windmill output, not to generate usable energy but simply to test and gauge his equipment.
He was attaching his windmill to his pickup truck to test blade designs, not to try to harness energy while driving.