If the question had asked what ailerons and rudder are then the answer would have been primary flying controls. But saying that the ailerons and rudder are primary flying controls is not the same as saying that primary flying controls are the ailerons and rudder.
The question asked what primary flying controls are. They are by definition, those flying controls which control rotations of an aircraft about its three axes.
This does not mean that everything which is able to control motions about the three axes must be a primary flying control. In most aircraft with underslung engines, any changes in thrust produce pitching motions. But this does not make the throttle a primary flying control.
Trim tabs, balance tabs, and servo tabs are all forms of aerodynamic balance. Their function is to control or modify the control hinge moments. These tabs are not intended to control the rotation of the aircraft about its three axes. They are intended to assist the pilot to move the primary flying controls to do so.