The filling-factor-dependent plateau-type dispersion of the long-wavelength magnetoplasmon in high-mobility two-dimensional electron system observed by Holland et al. [Phys. Rev. Lett. 93, 186804 (2004)10.1103/PhysRevLett.93.186804] can be explained by the well-established semiclassical dispersion, by adopting the electron reservoir hypothesis previously proposed in order to explain the integer quantum Hall effects.