After reading the above link, i interpret it like: if you have enough experience with the car and track, you should (especially at higher grip levels) run a more stiff chassis. But just look at the Worlds in Kissimee, most drivers were driving quite soft flex setup.
To come back to the theory of gaining maximum grip on an uneven surface: Carpet is very very soft compared to asphalt (literally :-D). And the bumpes are completely different (for example a kink in the carpet). The bumpes are maybe higher amplitude but at much lower frequency.
So the benefits of the soft chassis (lower wheel load alteration) doesn`t show up really, but the negative effect (less responsive to suspension setup changes) show up even more because of higher cornering g-forces.