When a
symmetrical airfoil is at zero lift it is also at zero alpha
and there is no twisting moment on the wing. When a normal positively cambered
airfoil is at zero lift it has to be at a small negative alpha to cancel out
the camber, which gives some lift at zero alpha.
The resultant pressure pattern round the airfoil gives some negative lift at
the front and some positive lift at the back - total lift zero but with a nose
down twisting moment.
By analogy an "upside down" negatively cambered airfoil will have a
residual nose up twist at zero lift, which will be at a small positive alpha.