They're not using it as a "verified source", except in that they're saying "this is the source."
It's actually not a bad approach. Prediction markets (which betting sites are just a form of) are often very good at reflecting what the current well-informed belief on a subject is, because people who make bets on ill-informed beliefs quickly end up not having money any more and thus not betting any more. It's just important to bear in mind that it's not literally saying "this is what's going to happen," it's saying "this is what well-informed people currently believe is going to happen based on current information."