In a recent paper [Journal of Physics: Condensed Matter 29, 475301 (2017)], Álvaro Díaz-Fernández et al. have considered electron states in quantum wells with band inversion. In IV-VI semiconductor heterojunctions with band-inversion, such as those made of Pb1-xSnx$Te or Pb1-xSnx$Se, interface states are properly described by a two-band model, predicting the appearance of a Dirac cone in single junctions. However, in quantum wells the interface dispersion is quadratic in momentum and the energy spectrum presents a gap. They show that the interface gap shrinks under an electric field parallel to the growth direction. Therefore, the interface gap can be dynamically tuned in experiments on double-gated quantum wells based on band-inverted compounds.