  The titular manuscript invites a description of the background of research on
MgB$_2$,whose unique electron-phonon coupling strength and record
superconducting T$_c$ for such a material at ambient pressure was demonstrated
by the combined efforts of several groups two decades ago in terms of
conventional but highly developed Migdal-Eliashberg theory. This Comment
provides some of the basic features of the theoretical understanding of MgB$_2$
and draws contrasts with the model of Yu {\it et al.}
