Your whole theory is based on the assumption that volume must remain constant when gaining girth. There is no length gain if your penis doesn’t get longer, that’s just common sense. If the volume of your penis increases but the length remains the same, the only thing you increased is girth.
I agree. I have made both length & girth gains simultaneously. One never necessarily excluded the other. I have lost some girth when I eliminated jelqing to focus solely on stretching, but that’s because of a radical change in my workouts, not because of shifting differences in my volumetric size.