Mountain or Forest Tunnel isn't new. One of the first shit Shin and Co Explored is a fucking Mountain Tunnel which was guided by Ten. Heki knows that. Their Hideout after that Mountain tunnel is a fucking Forest tunnel, Which Heki also fucking knows that's literally the very first part of the manga. When the emperor was still running away when he first met Shin and Ten. If Qin has those Tunnels existing on their land why wouldn't Zhao have the same tunnel used in their land. Why would Heki be surprise by this tunnels when this are common shit that they exploit as an Army.