We are talking about the wing twisting moment. On a symmetrical airfoil at zero lift, zero alpha, the pressure patterns top and bottom are the same, so there is no residual tendency for the wing to twist - zero moment.

On a cambered airfoil you have to go nose down alpha to get zero lift. In this attitude the zero lift is the sum of some positive lift at the back of the airfoil and some negative lift at the front. This pressure pattern produces a residual nose down twisting moment - even though there is no overall lift.