  In a recent paper by S. Moroz, C. Hoyos, and L. Radzihovsky [Phys. Rev. B 91,
195409 (2015)], it is claimed that the conductivity at low frequency $\omega$
and small wavevector $q$ along the edge of a quantum Hall (QH) system (that
possesses Galilean invariance along the edge) contains a universal contribution
of order $q^2$ that is determined by the orbital spin per particle in the bulk
of the system, or alternatively by the shift of the ground state. (These
quantities are known to be related to the Hall viscosity of the bulk.) In this
Comment we calculate the real part of the conductivity, integrated over
$\omega$, in this regime for the edge of a system of non-interacting electrons
filling either the lowest, or the lowest $\nu$ ($\nu=1$, $2$, . . .), Landau
level(s), and show that the $q^2$ term is non-universal and depends on details
of the confining potential at the edge. In the special case of a linear
potential, a form similar to the prediction is obtained, it is possible that
this corrected form of the prediction may also hold for fractional QH states in
systems with special forms of interactions between electrons.
