Still doesn't make it an invasion. If they drone striked and thus killed Majuro it would not be an invasion. It would be an assassination.

Invasion in this context has a specific meaning. The bet on the market would have been done with this specific meaning in mind.

No invasion, means no payout.

It would be like making a bet where someone scores in Football/Soccer from a penalty, but in the game they score from a free kick outside the penalty box. You wouldn't pay out on the bet, because a penalty is not a free kick even though they are similar and had the same result.