A sufficiently large market is indistinguishable from Brownian motion.

That's a model.

From Schlock Mercenary (quoted from memory, may be inexact):

"You cannot see the future. All we are given is the present."

"Of course. But if you look closely at the present, you can find loose bits of the future just laying around."

Not really. Not all players in prediction markets are rational players. A good chunk of it are there for entertainment, and analyze things incorrectly; you can take the other side of those trades, and you won't need to predict the future.

Deciding that someone else's prediction is wrong is a prediction in and of itself.