Hi!
The torque is the action that creates rotation of the parts around the given axis.
The pressure moments in x and z directions are reported as acting in the center of rotation, and the torque compensates for the translation of the forces. Thus, the calculation you outline is not quite correct.
Instead, you would need to take the resultant force vector at each cell in the surface and use the cross product with a radius vector from the rotation axis. Then, you can project the moment to the axis with the dot product.