He broke one of the rules. He was promoting piracy, even if it was a joke.
The rules are stickied on the top of the Off Topic board which he was in. They make it clear not to be discussing piracy, which includes promoting it.
It's not a valid excuse to not know the rules.
They are written up in stickied topics in all the boards. Users are expected to have read them, understand them, and follow them. Not doing so doesn't let you get away with breaking them.
If you're in town and you break the law without knowing it a police officer can still arrest you for it. It's just not an excuse to be unaware of the law, and that really does apply here too.