It's hard to tell with you sometimes.
With most people it's easy: if it's funny, it's a joke...
He could earn some buck either by telling Stunna about the bug or by moderating his luck, but the way he did he got no winnings
Because sites often don't pay well for reported exploits. Just yesterday a guy was posting on the SatoshiDice thread about how he can withdraw his balance twice to double his bankroll, but they don't believe him and so won't pay him.
Maybe he has been using the exploit in a less obvious way for a while now on other accounts and has already won all the coins he wanted to win. This could be his fun way of "reporting" it.