If that’s what you took away from the video, you should watch it again. But it’s true that this video doesn’t explain how the car speeds up from below-wind-speed to above-wind-speed.
When the car is stationary, the wind is pushing on the car itself. The wind would actually put some force on the fan blades to try and make them turn clockwise, but the forward motion of the car, and the coupling of the wheels to the fan blades, actually makes them turn in the opposite direction.
As the car moves faster, the fan blades move faster. They never slow down or switch directions as the car transitions from moving slower than the wind to faster than the wind.
