Comment on "Absence of Topological Protection of the Interface States in ${\mathbb{Z}}_2$ Photonic Crystals"

In the Letter, Xu et al. reported that edge modes disappear in the expanded structure of Wu-Hu model characterized by ${\mathbb{Z}}_2$ topological index, while appear in the trivial shrunken structure, when the edge cuts through the hexagonal unit cell. They then concluded that these edge modes are defect modes lacking topological protection. Unfortunately, their approach is not justified, rendering the conclusion unsolid.