If I got the essence of what your saying right, that rider flexibility affects positioning, then yes, I agree with that. However, I'm not so sure that it affects frame sizing all that dramatically, or that Cobb would consider A and B style backs 'positions'.

My basic thought is that flexibility and pelvic positioning allow riders to get into lower, more comfortable and aero positions, but that frame size is still determined by a complex recipe of things like riding style, intended use, body type, frame geometry, etc. Within that range of frames that work, then, flexibility and pelvis angle help achieve the most more aero position. Rider ability to go lower might, in some cases, suggest a smaller frame (lower stack), but I don't think simply being flexible would recommend a larger frame.

To that point, Cobb's A & B back thing refers to rider body types, and how much back flexibility they have. Recognizing the styles informs him on how he should attack fitment for the max aero and comfort benefits for a given rider on a given bike. I don't think Cobb would suggest B backs should aspire or practice to be As, and I wonder if he'd think it's even possible. Similarly, Fizik's Chameleon/Snake/Bull thing recognizes flexibility and how that impacts riders' likely pelvic position on the saddle; it's a fit adjustment tool, not an aspirational training goal.

I'm a Cobb believer, and pelvic rolling worked for me in getting lower and more comfy. But rather than suggesting I go longer, it actually shortened my overall reach and allowed me to keep my weight centered on the bike for excellent handling feel and control.

Anyway, I guess the point that I wanted to make was that while flexibility and pelvic tilt give greater range of fit setups, I think it's a very wrong approach to try to train in flexibility in order to make a larger frame work; I just don't see the point, and think that for sports road riding, the smallest fitting frame is preferable in all ways.