I don't know what happened. I arrived in Athens at 11 PM and had no place to stay. My reservation was still valid and not cancelled. I was just told there was no availability, whatever that means. If there had been a cancellation I would have made other arrangements.
I called Airbnb from the airport. Their concern seemed to be for Airbnb procedures. They basically hung up on me to check with Giannis, who was incommunicado. The bottom line was that I had no place and no help (or SIM card at the airport at midnight) so ended up taking a bus downtown and trying my luck with hotels.
Better communication all around would have helped.