It's extremely inaccurate to say 80% of the gamblers were convinced because that's not how it works at all. Each bet has someone on the other side of it - 50% of the people bet on Bolsanaro and 50% against.

If you bet on 20% chances and they happen 30% of the time you make money. The payoffs are such that you win by being a better predictor than the market. I'm fine criticizing it as a gambling blight (not to mention a corruption payoff) but it is also a prediction market.