Earlier quoted context omitted.
Where the power comes from is easy: the wind gets slowed down. Of course the nature of wind is such that you could never measure this, but the point is that when the propellor pushes against the air, that air's velocity relative to the ground is decreased while the vehicle's is increased. The vehicle leaves behind it an imperceptible region of less-energetic air and takes that energy with it.
Now I think I'm beginning to understand... At first, wind propels the vehicle, moving it forward and turning its wheels. Its wheels turn the propeller which slows down the air, even the air in front of it. The energy lost by the air is now the energy that moves the vehicle! Indeed! There is no requirement that the vehicle must be slower than the wind! I think understood it!!!
Suppose the vehicle is facing the +x direction. Suppose the wind is travelling at +10m/s relative to the ground. Suppose the (experimentally determined) maximum velocity of the vehicle under these conditions is +12m/s. Now consider an inertial reference frame which, compared to the ground's reference frame, is moving at +11m/s. (I.e., pick the frame in which the ground moves at -11m/s.) Now let the vehicle run. There will come an instant when the vehicle is stationary in this reference frame (when the vehicle has accelerated to +11m/s relative to the ground, not yet having reached its maximum velocity).
Consider the instantaneous change in kinetic energy at this instant. The air about to be pushed by the propeller has velocity -1m/s and will accelerate in the -x direction. The vehicle has velocity 0m/s and will accelerate in the +x direction. In both cases, the kinetic energy is actually increasing! So in this frame, where does the increasing kinetic energy of the air-vehicle system come from?!
The answer is the ground. It has velocity -11m/s. When it pushes against the contact point of the wheels, it is therefore pushing the contact point in the -x direction. But this means the contact point is pushing against the ground in the +x direction. And therefore the ground is undergoing a minuscule +x acceleration. That acceleration is therefore decreasing the kinetic energy of the ground — by a lot, because of the mass factor.
And this is why we say the energy is derived from the difference in velocity between the ground and the wind — because depending on your inertial reference frame, the kinetic energy might be coming from one or the other. In the frame of reference of the vehicle, it starts out coming from both, but when the (relative) direction of the wind shifts (so that relative to the ground the vehicle is travelling faster than the wind), the kinetic energy starts coming from the ground only, and the vehicle is forced to transmit some of it to the air to keep moving. But this might actually be the nicest explanation of how the vehicle outpaces the wind: in the faster-than-the-wind regime, the vehicle is transmitting energy from the ground into the air — which is exactly what you would expect to happen, given the mechanical linkage between the wheels and the propeller! — but some of this energy bleeds off into increasing the velocity of the vehicle itself, because it's on wheels and that's what happens when wheeled vehicles push against something behind them, no matter what the ground happens to be doing (such as moving backwards like a treadmill, in this frame of reference).
Disclaimer: I did not do well in college-level Mechanics. But I think I have convinced myself of the above explanation, just barely.