The "advance" we speak of is using the engine as a point of reference.
The problem is, we are now using the distributor as a point of reference. So if it's true that the engine advances the distributor, then, isn't it also true that the distributor retards the engine?
The link between the upper distributor and the engine is the mechanical advance. This link is what changes the relationship between the engine and upper distributor. The upper distributor stays constant with itself, but not the engine.