That isn't guaranteed, because if so you can always do the opposite of what you think and you will overperform. Even if you think you know something, you are, at best, still being random. Anything perceived decrease in return from taking actions is itself just chance (and confirmation bias), because otherwise you could inverse it.

> Even if you think you know something, you are, at best, still being random

Technically true. But somehow in practice you are random in worse ways. Psychology of most people makes them generate very bad randomness.

But you are right. What I said is just good first approximation. Sometimes you can do better. For example listening to most popular financial influencers and doing exactly opposite of what they recommend gives slightly better returns (it was researched). I don't quite remember if better than fully random though or just better than following their advice.