Why Graf Zeppelin's guns do not make a carrier duel sensible

Would Graf Zeppelin's heavy secondary battery let her defeat another carrier in a surface engagement?

Not reliably; escorts and torpedoes remain serious threats, and a carrier-on-carrier gun duel requires an implausible isolation of both ships.

A gun duel between Graf Zeppelin and another isolated carrier requires an implausible chain of events. Even HMS Glorious, separated from stronger support by poor command decisions, still had two destroyers capable of making torpedo attacks. Graf Zeppelin would have to fight those escorts while manoeuvring a large and comparatively unwieldy carrier. Her secondary guns do not turn the encounter into a favourable one because carrier warfare is normally conducted by groups rather than solitary hulls.