I think he is still not describing the Black Bird physics correctly. Each propeller blade is the sail of an Americas Cup AC-75 going downwind with the chain drive from the wheels moving the sails the way a sailing craft would move downwind. The craft moves faster than the wind, because its "sails" (i.e. propeller blades) are accelerated to a VMG greater than the wind speed and the cart is attached to those "sails".
The AC-75 hydrofoil craft move downwind faster than the wind just as an ice-yacht. The innovative idea with this cart is, that the sails do not move linearly but rotate about a pivot point. As pointed out earlier one important point is that you have to get the transmission ratio from the wheels to the propeller right so that the propeller blades move fast enough to generate that relative wind over the blades that moves the whole thing.