One thing to keep in mind when reading about any "ZKP protocol" is that on its own the term has the same inherent vagueness as "end-to-end encrypted" - especially when like here there are more than two parties concerned to solve a single verification.

What information is not disclosed to whom? In what way is it ZK?

One example is Googles "zero-knowledge age verification" where AFAICT, Google still has full insight into all the sensitive data and metadata. It's not like they inherently need to be the designated middleman but that is how the scheme is designed. Therefore I find it ingeniously marketed. A bit like saying "Facebook Messenger protects all your messages with end-to-end encryption", which is arguably technically true but misleading and not an honest statement.