  The authors of a recent paper evidently take the view that the whole of
progress made toward a theoretical understanding of the physics of quantum spin
liquids (QSL) is associated with models of the kind proposed and applied in
their present work. As motivation for this work, they observe that in contrast
to existing theoretical models of both gapped and gapless spin liquids, which
give rise to sharp dispersive features in the dynamic structure factor, the
measured dynamic structure factor reveals an excitation continuum that is
remarkably flat as a function of frequency. They go on to assert that "so far,
the only theoretical model for a spin liquid state on the kagome lattice which
naturally gives rise to a flat excitation band at low energies consists of the
$Z_2$ spin liquids". Here we point out that there already exists a different
and demonstrably successful approach to the QSL problem that does naturally
feature a flat band.
