I don't know that's true, although I don't know it's false either. I have seen on the net that guys sometimes use a 50cc pump with the small cam, but I am not sure it is the same output as a 30cc pump. Although the sweep and rise would be determined by the cam, seems to me the dimensions of the 50cc pump might do something different.
Regardless, I have done a ton of Holley work, and I don't use those 50cc pumps. I honestly have never found a need for one. The only time I can imagine that I wouldn't be able to tune it with cam and squirter would be for an alky carb when it needed much more volume or if there is a significant issue in the transition and main circuits.