I don't quite follow this. Surely each poker player who raises thinks that they will take the pot and the other players will lose. So they don't each think that the raise is mutually beneficial. Player A thinks that the raise is beneficial for player A and detrimental for player B, and player B thinks that their call is beneficial to player B and detrimental to player A. Which seems like the definition of 0 sum.

Compare this to something like trading apples and oranges, where one person gets an orange (maybe they are tired of apples) and the other gets an apple (maybe they are tired of oranges). Both gets something they want in exchange for something they don't want.