People have heard of the Blackbird vehicle that can exceed wind speed. But active control is required to control the pitch of the blades. It's an either/or process, either the wind is pushing the vehicle, or the vehicle is pushing the wind back. From an article on it,
"getting going in the first place might seem like a chicken and egg problem. Which comes first, the wheels turning or the prop turning? Cavallaro says that is not an issue. He has some control over the pitch of the blades, and by flattening them at the start, the car gets the push that it needs."
Are they wrong? Is controlling the pitch of the blade not needed for acceleration from rest? How would momentum add up to exceed wind speed in a passive system?
If there is evidence, please link to it.
Whether one would call this fully wind powered is another question (like if a gliding bird is wind powered, why doesn't a dead bird fly), and whether leaving this info out is a fair bet for the prof who bet against veritasium on it. The model made that does something similar needed to be accelerated by hand.
Edit: if you say this is an engineering question, then you would be saying the inventors themselves are wrong, because they say it's not possible.
Edit: this is about a constant wind speed. A higher initial wind speed of course could make an object move faster than the slower wind speed after.
Edit: if it works sometimes without a variable pitch, the question is why not always? What are the varying conditions?