The standard you linked literally talks about: "High Impact BES Cyber Systems with External Routable Connectivity" and "Remote Access Management" for "High Impact BES Cyber Systems". That explicitly indicates non-airgapped critical systems. Furthermore, the proscribed auditing specifically spells out "network diagrams or architecture documents" as good evidence. Obviously, that is a high level document, but I see nothing to indicate robustness against state-level actors which are a expected threat.