It's not. Before this "experiment" began, there was already a balance of around 18 BTC in this address. So there were already coins to move around to prove he h

It's not. Before this "experiment" began, there was already a balance of around 18 BTC in this address. So there were already coins to move around to prove he held the private key. But lets say he does have the key, and the balance was 0, the easiest way would be to just sign a message with the private key. Unless he does not have the key or, he doesn't know how to sign a message.