I struggled with the sentence cause, at the same time, creating one global centralized authentication source creates the opposite of privacy in the sense of anonymity. Certainly OT wrt the actual content of the work...
As an authorization system Zanzibar focuses on: can agent A (identified through some means) perform action X on object Y. It isn't about deciding whether an arbitrary actor is agent A but proscribing what actions agent A can perform against the universe of all possible objects (which likewise are referenced abstractly and not stored within the system itself).
The knowledge that A could do X on Y is information that might be disclosed (and thus entails some privacy risk)... but inherently doesn't reveal: anything about the identity of A; whether A has ever done X; or what Y's contents are or what it represents.
On the other hand, perhaps you mean that because membership in sets of users is also stored within it (via a sort of "is member of" permission) you can use that to de-anonymize who a given actor is. This might work but it assumes you can uniquely derive which agent from a set of abstract agents represents that individual and that you extrinsically something about the person being the only person in this specific set of sets.