I try to understand "intellectual curiosity" here but I just find it very off-putting to bet on whether millions will die, no matter how sound the arguments of both sides might be.
The winner of the bet does not profit personally, so it's not really a bet. They use the term "bet" because it's easy for people to understand. And it makes it more interesting than alternatives like "prediction & rebuttal".
Long Bets and the Long Now Foundation try to get people thinking about the future so we can make it better.