I don't know if the guy was scammer but certainly it proves all the people wrong who said that site to be scam i think.
Lol why is that? Because they refunded the BTC?
First of all, it could be as well a PR stunt. But even if the guy is real, CMC would simply calculate whether they're better off refunding him (building positive impression) and carry on with their ponzi, or keep the 20 BTC and scare away any potential new customers. Simple math, they chose the option they believe is more profitable.
That proves nothing.