[drdeca]

It sounds like you are maybe describing a dominant strategy? Oh, wait, no, you are saying that if you play the Nash equilibrium, then no other strategy among opponents will do better against it than the Nash equilibrium would, and therefore (as the game is zero sum) the worst their choice of strategy can make you get on average, is breaking even?

Ok, now that I (I think) understand your comment: I don't think you have to know exactly what non-Nash strategy someone is playing in order to exploit it. I don't think trying to estimate how someone is likely deviating from the Nash equilibrium, in order to try to exploit it, is necessarily a mistake. I think it could be feasible for someone to get better returns on average by noticing off-Nash patterns of play in other players, than playing Nash regardless would? (Not that I could win this way. I couldn't.)

[chii]

But isnt the definition of the nash equilibirum the strategy where no deviation from it is going to afford you a better outcome?

[drdeca]

A Nash equilibrium is a pair of strategies (or, one strategy for each player) such that no player can get a better result on average from deviating from it.

If one player isn’t playing a strategy that is part of any Nash equilibrium, then the best response might also not be part of any Nash equilibrium.

If all other players are playing a strategy from a given Nash equilibrium, then you can’t do better (in expectation) than you would if you were to play the strategy for you in that Nash equilibrium.

(A game may have multiple Nash equilibria. Possibly one such equilibrium could be better for you (or for everyone) than another.)