your IQ must be high if you can't tell if it's a joke or not
It's hard to tell with you sometimes.
With most people it's easy: if it's funny, it's a joke...

Why people abuse exploits like this?
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.
the way he used the exploit was dumb, betting large amounts that can be seen by other people hmmmm... Obvious way to get caught cheating
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.