You are %100 correct. But the difficulty is understanding how the dual vacuum advance works until you take one apart.
The retard side of the dual vacuum advance is not physically connected to the distributor point swivel plate (inside distributor) or the arm that is attached to the swivel plate. It does absolutely nothing if the advance side is receiving vacuum. All it does is move the rest position (inside the vacuum advance unit) of the vacuum advance side depending on the what condition the engine is in.
The higher the vacuum in the inlet manifold the more the retard rest position is moved towards the retarded side.
The amount of timing advance at idle is controlled by how much vacuum is supplied to the advance side of the vacuum advance unit, and this depends on if the vacuum is supplied from the intake manifold or the carburettor port.
(Note- The ported source at the carburettor, does not have any vacuum when the carburettor butterflies are closed.)
And whether the vacuum advance is supplied with vacuum from the carburettor port or the inlet manifold depends on the temperature controlled vacuum switch, which in turn depends on engine temperature.
So if the temperature controlled vacuum switch, switches vacuum supply to the carburettor port at idle, where there is no vacuum, the advance side will come to rest in the more retarded position.
Sorry, it’s not a very good explanation, very difficult to explain. I had to take a dual vacuum advance unit a part and then follow the vacuum diagrams before I could get it. Ask again if troubled.
Peter 