How does one "interpret" public knowledge to time bets so accurately right before Trump's announcements?

I mean that's why insiders are being investigated right?

But sometimes the answer is more difficult than it seems. Is a mid level military officer an insider? If you overheard a conversation on Capitol Hill are you an insider?

First, you’re describing the insider trading that is not permitted.

Second, the majority of prediction markets are predicting utterly mundane things like sports. The tiny number of news grabbing markets are not representative.