Has this actually happened? Does this prove anything if it did, given that winds at altitude and ground level are vastly different?

It is impossible for any existing sailboat to have a downwind component that is faster than the wind. If it were possible, you could sail the boat with no wind at all. (This argument does not apply to the ground vehicle under discussion.)

Will you at least agree that it is impossible to sail with the boat pointed directly downwind faster than the wind in a conventional sailboat (including racing sailboats)?

A sailboat can reach faster than the wind because the mass of the wind is greater than the mass of the sailboat, and the energy in the wind is transferred to the boat. Moving downwind faster than the wind is very different, and requires a different mechanism. And you cannot use apparent wind to explain moving downwind faster than the wind, because the apparent wind would be in the opposite direction.