There is no insider trading on a bet, it's kind of the point.

Would you bet a large amount of money without some insider information?

Insider trading on prediction markets is the whole point. They don't exist to provide a fair platform for normal people to make money, they exist to create accurate predictions by providing a monetary incentive for people to be correct. Whether "correct" means that you were just lucky, that you had insider knowledge, or that you were able to influence the result, is irrelevant.

If I know the outcome, why would I bet largw before the outcome is to be publicly revealed?

I just wait and bet only if the market is wrong when the reveal is imminent.

Isn't this a realy good money laundering avenue? I sell you guaranteed outcomes, for a fee ofc.