I don't know every factor that goes into building density on the film (I don't know if anyone truly understands it 100%). So any interaction between existing fog exposure and new light just falls into the area I don't fully understand. I'm sure the actual physics behind the exposure of silver salts are very complex. But from a sensitivity standpoint it's all just bundled together into a single density reading. Maybe this is responsible for some of the non linear responses we see, but I don't know.

Another way of saying it is: does it matter how the density got there, other than it was a result of light hitting the film? Either way the curve shows density vs light exposure. So unless you are interested in the physics of how density builds up it isn't important (not that it isn't important in an absolute sense).

I guess one test you could do is to look at the curves of film with low fog and the same film after aging with higher fog. If the curve shape shifted it may give you a hint that there is an effect. It does seem to cause a loss in film speed just based on casual observations.