When the rudder is used to bank an aircraft, the slight differential in speed between the advancing wing panel and the retreating one is not enough to produce the difference in lift needed to bank the aircraft...unless the wings are extraordinarily long. The reason the plane will bank is because of the dihedral in the wing panels. When the plane is yawed, say to the left, the right wing is advanced ahead of the left wing. The wing's dihedral then causes an increase in the angle of attack vis-a-vis the relative wind. The left wing will
"see" a reduction in AOA to the relative wind. This causes the bank.

A swept-wing aircraft will experience a similar effect because when the aircraft is yawed, one wing now has "more" span exposed to the relative wind, and one wing has "less" span exposed. This also causes a differential in lift, which causes the bank.

If the speed of the advancing/retreating wing panels was the main reason, then low-wing models with zero dihedral, such as the Top Flite Contender would still have positive roll couple with yaw. They do not. They act just the opposite, with right yaw producing zero or left bank. The adverse roll couple is caused by the same reason as above, except the advancing wing now "sees" less of an AOA to the relative wind, and the retreating wing "sees" more.

All of the above can be easily seen visually if you use a high-wing model with dihedral and a fixed point from which to view. Set the model dead-on and the viewpoint ahead of the model such that both wings have an equal appearance. Now yaw the wing a few degrees one way. From that same viewpoint you'll see more of the underside of one wing that the other. The wind "sees" this as if the entire wing panel pivoted its leading edge upward. The increased AOA causes more lift on that side. You'll see more of the upper side of the opposite wing panel. That means that the apparent AOA to the relative wind has decreased, and the lift produced is reduced.

Wolfgang Langeweische's book, "Stick and Rudder", presents a very clear picture of how an airplane flies, and how the pilot controls it. as any book I've ever read. Please be advised that he presents all of this in a phenomenological manner, and not in a truly scientific way. His book is concerned about having clear mental pictures of what's going on so that the pilot can do a better job of controlling the aircraft. "True" aerodynamics he leaves up to the designers and scientists. An invaluable book for full-size as well as model pilots.