What's important here is that there are two mediums (air & ground) with a speed differential between them. Typically when you want to use this differential you make a windmill. Anchor your generator to the ground and extract energy from the moving air. In essence, what they've done here is anchor the generator to the air and extract energy from the passing ground. Quite a bit less efficient, I'm sure, but doable.
Lets work it out in the ideal situation. Begin with a vehicle traveling same speed as the wind. The frictionless wheels are currently free-wheeling on the ground with no resistance. Consider this 'anchored to the air', since we are motionless relative to the surrounding air. Now, from this perspective the ground is moving past us and our tire is spinning in it's medium like a free-wheeling windmill blade spinning in it's medium. Next, you put a generator on your spinning wheel / fanblade. Now you're extracting energy from the system. You can put it in a battery, or you can use it to push yourself forward. Since it's not windy in the vehicle, you could even mill flour with it!