If Alice can share notes with Bob in a manner where the service itself cannot determine that Alice and Bob have any connection at all, then go ahead and market that as Zero Knowledge. If the service can determine who is talking to who, though either passive or active means, then it is disingenuous to claim the service is Zero Knowledge.

Unless the anonymity set is very small or bandwidth is infinite, accomplishing metadata privacy requires some kind of zero knowledge proof.

I don't think such systems necessarily require E2E encryption. If the term zero knowledge continues to be misused, it's going to cause more confusion later when services get around to protecting metadata.