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Wind can't blow something downwind faster than the wind speed, but that's not what's happening. The wind is generating energy through the ground into the wheels and the wheels transmit it back into the propeller which is able to add it back into the air. It doesn't take much to add.
This wouldn't be possible without the ground. You couldn't do it on the water with a sail boat or in the air with a glider (assuming level flight).
Side note, now thinking about that boat. I wonder if this same principle could be used to propel a boat down a river faster than the current flows?
The way it works in my mind (meaning I'm not doing that math which shows it works) is thinking of it more as a wind energy concentrator than a sail. Energy is still conserved.
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