How does some video he made over a year ago any indication to his motivation for starting a pirate "bet"? The only reason this was an epic "prank" instead of a win for him, is that he was dumb enough to side with pirate.
MNW knows how to signal when he's trolling and when he's not. Most of the time he's trolling. Occasionally he gets serious, eg when he accepted pre-orders for the magazine. During his bet, he made every attempt to signal that it was a genuine offer. Furthermore, if it was a prank there was nobody but himself to enjoy it (pranks are funny because you can laugh with witnesses who were in on the joke). It only started to seem like a bad joke in the third week after it was clear pirate wasn't going to pay, and he started accepting more bets way beyond the original 10k cap. Even at that point, MNW was very quiet and any signs that it was prank were extremely muted.