"If one were to change nothing but the concentration of the developer I assure you the contrast of the negative would be changed."

"I think it's safe to say that for all practical purposes developer concentration affects contrast."

Yes, if nothing else is changed (i.e. if you developed the more concentrated developer for the same time as the weaker concentration). However, when manufacturers contemplate that developers may be used at different concentrations (e.g. Rodinal, HC110, D76) they also provide different times for each concentration. For example, the label on the Rodinal bottle provides for different concentrations and different times depending on the concentration that is used. If the instructions are followed when the concentration is increased then the development time is reduced. When that's done there is no change in contrast between the longer time with the weaker concentration and the shorter time with the stronger concentration. That's what I meant when I said that changing the concentration doesn't affect the contrast.

I agree that if Rodinal, for example, was for some reason used at the 1-10 concentration and developed for the 1-50 time, or if D76 was used undiluted but developed for the 1-1 time, the contrast would be different than if the recommended times for the stronger concentrations were used. However, when that's done it isn't the change in concentration by itself that increases the contrast, it's the combination of changing the concentration and using a longer than normal development time for that concentration. That's what I meant when I said that changing concentration by itself doesn't change contrast.