You're talking about roll effect and roll stability. That's not roll couple.

A low wing without dihedral has negative roll stability. Add dihedral and you can balance out that roll instability. And the airplane can be designed to have neutral roll stability by giving a low wing the right amount of dihedral. But if it's got dihedral, and you yaw it with the rudder, it will tend to roll in the direction of the yaw, which is rudder roll couple.