In [Phys. Rev. Lett. 98, 103902 (2007)], Tarasevitch et al. demonstrate the existence of two generation mechanisms for laser high-order harmonics from overdense plasmas. One of these mechanisms leads to harmonics with frequencies up to the maximum plasma frequency of the target and occurs even at nonrelativistic laser intensities. We show that the mechanism responsible for these harmonics is coherent wake emission (CWE), a process that significantly differs from thequalitative model proposed by these authors, and it leads toa different interpretation of several essential features of this emission.