But there are those with zero volume wagered but also won the prize, what makes them eligible for such prize?
I doubt those with zero wager will win the prize at the end of the contest, as far as I can see is that the leaderboard showing all players who are registered with the provided link. It will be wasting money for the casino to give free money for players who did nothing. However, this is still on going contest and there are still many days till the end of the contest so it may changed later without players with zero volume to win the prize.