Right, I see what you're saying, but this is what I'm disputing - in two player games, what you wrote is true, but those properties of Nash equilibria don't generalise.

When there are more players, there can be multiple Nash equilibria, and (unlike the two player case) combinations of equilibrium strategies may no longer be an equilibrium strategy. So it's no longer true that you cannot be exploited, because that depends on other player's strategies too, and you cannot control those.

(See this paper for instance: https://webdocs.cs.ualberta.ca/~games/poker/publications/AAM...)