Impossible to use Application Default Credentials (ADC) + Admin Directory API (@googleapis/admin)? #3634

I've noticed that calling Admin Directory API seems to work only when authenticating with a service account key, not with application default credentials (ADC).

GCP APIs can be called with ADC, but for some reason it seems that Admin Directory API (i.e. Cloud Identity / Workspace resources) cannot be called with them, at least not without some special arrangements(?).

Is there a working code example to get Directory calls to work with ADC? If not, could it be clearly mentioned at least in the repo readme that ADC usage is not possible with Directory APIs (preferably with a justification why)?