If I'm an insider with 100% confidence, I'll take all offers at a certain price as long as I can afford it. Similar story for lower levels of confidence (but still inside info). There won't necessarily be any left for you to copy at a viable price.
Because there's always some uncertainty and capital limits. But the uncertainty about the outcome is itself inside info, and that's compounded with your own uncertainty about the insider as a copy trader. So the insider will empty out certain price levels only, and your certainty is strictly less than theirs, meaning you have even fewer viable levels to buy.
therefore, the polymarket betting odds will reflect the truth - even if that info is a secret that nobody else but the insider knows. And if this is the case, then even an outsider could make use of the odds as a source of info which would ensure that market efficiency (which is about the flow of information) is high.
If you believe Polymarket as a serious source of truth, consider that somebody manipulated "Will Jesus Christ return before 2027?" because there was a secondary market on whether that market will rise above 5%. Which defeats the whole idea that the betting odds will reflect the truth. Also even pre-manipulation I don't think a 2% chance that Jesus will return was reflective of truth.
The issue comes from situations where the insiders can alter the answer to help their own bets. The simple example is the bet on how long a press conference will be: It's a ridiculous bet when the person giving said press conference can bet and fleece the market.
Will X country invade another before or after day X? A large enough market changes the answer, as the agent can change the decision. And we can see this kind of thing in many interesting questions.
These are not secret divinations though, the participants know this and price it in or otherwise allow it to determine which markets they participate in.
That someone with inside information will e.g. make 500% while those late to the party e.g. only get 10%? (of course your example is not very realistic to begin with)
Has there ever been any documented circumstance where significant inside information became public and known thanks to a trade? Most often, the trade is made at the last minute, and the information gets subsequently revealed anyway. And it's impossible to tell whether somebody is an inside trader, a wealthy gambling addict making a stupid decision, or hypothetically a foreign agent pretending to be an inside trader to make people believe in a particular outcome.
It's impossible to know anything for certain; almost everything is probabilistic.
Also I'm not sure how to interpret your criteria because timing matters, I don't think saying 'it gets revealed in the end' is very meaningful.
Anyway, on Polymarket specifically, sure, military strikes are a common one. Seems like a useful signal to go hide in the basement. Outside Polymarket, there were insider trades in 2008 that I'm sure were useful.
No vigilant insider is making a series of "single market predictions with high accuracy" on the same account. They would make unlinkable bets on fresh accounts.
