The rumble ball distrupts the wavelength of zoan types, at least by what chopper said. However, aside from dalton, there are no other df on the island( that we are aware of), so it's a guess on his part. I also believe hes not the only one to discover this "wavelength" property. The gun dog and elephant sword as well as awakened zoan types are the result of experiments on zoan types. There has been no stated method as of yet so it's only my guess.