Do we agree that when the wind is blowing that there is energy available to a stationary device?
There is a potential energy available yes that will be 0.5 * vehicle mass * (wind speed)^2
That potential energy can be converted in to kinetic energy
This is true for a vehicle with no energy storage device like a sail type vehicle.
This type of vehicle can drive directly down wind up to wind speed (depends on friction losses) and can not drive directly upwind at any speed.
Does this change if the device is moving? There is still a speed difference between the wind and the ground, regardless of the speed and direction of the device.
In the case where the device is moving directly downwind at windspeed, there is zero apparent wind (a sailing term, wind relative to the boat), but there is groundspeed, and the available energy still exists.
If vehicle is not moving there is no power available just a static force.
Power available to a vehicle moving directly down wind is this: 0.5 * air density * area * (w-v)^3 where w - wind speed and v- vehicle speed
From this you see that vehicle speed can be any as long as it is between zero and wind speed since when vehicle is at wind speed no wind power is available.
Accept this, and there is no need for energy storage, "slip-stick hysteresis", or any of that stuff. All that remains is to devise a method of extracting energy from the air/ground speed difference. This has been demonstrated.
If you want to exceed wind speed directly down wind then you need an energy storage device so that you can store energy while below wind speed and then use the stored energy to exceed wind speed.
Yes what was demonstrated was a vehicle with an energy storage device (pressure differential) and that can exceed wind speed. The test was incomplete else it will have showed how vehicle will start to slow down below wind speed as stored energy was used up.
It is fairly frustrating to see this disregard for energy conservation on an electronics forum.
If you have electrical knowledge think about this:
You have a 10V 3A CC-CV lab power supply and you can not output higher voltage or current using a device that has no energy storage device same for a negative voltage relative to ground/GND
You will need to use either inductors or capacitors or both in order to build a DC-DC boost converter. Using just resistor dividers you can get any voltage as long as it is between 0V and 10V to get more or less (negative) you need an energy storage device.