In the above link, what is the difference between artifact metadata and artifact repository? I feel like I am missing knowledge of some of the terminology.
The artifact repository contains the artifact metadata + some properties.
The pgp.publicKeys property can be set either as a repository property or as an artifact property. The former has the advantage of sharing the key for all artifacts, the later of the advantage of being attached onto the artifact metadata so it's supposed to be copied if you mirror the artifact with usual p2 mechanisms.