  In the recent work "Non-reciprocal topological solitons in active
metamaterials" (see arXiv:2312.03544v1), for an analytical understanding of the
system under consideration, the authors derive an ordinary differential
equation for the sine-Gordon (anti)soliton velocity, with the perturbation
theory in the adiabatic approximation, via the inverse scattering transform
formalism, see Eq. (3) in their work. Here we note that the latter equation for
the (anti)soliton velocity also follows from an energy balance approach.
