When you're traveling downwind at the wind speed, like the article says, there's no more apparent wind that you can harness to go any faster. That much is uncon

When you're traveling downwind at the wind speed, like the article says, there's no more apparent wind that you can harness to go any faster. That much is uncontroversial. But any type of "sailing" that isn't directly downwind depends on the speed difference between the air and the surface (land or water). If you have this type of speed differential your vehicle speed is limited only by friction and sail size. What they've done is applied this principle to direct-downwind sailing. While you can't get any energy from apparent wind in this situation, you can use the speed of the wind (and the vehicle) relative to the ground to extract some energy, which can then be used to increase the speed of the vehicle. From an energy perspective, it's not unlike a having lever fixed to the ground at one end, with a sail in the middle, and the vehicle at the other end. In this situation the vehicle can clearly travel faster than the sail without assuming any kind of perpetual motion.