I think it’s not correct when #ActivityPub thought leaders consider JSON-LD a schema definition language. I don't think it's an effective alternative to JSON Schema (for JSON) or SHACL or ShEx (for RDF). JSON-LD can’t be used to describe structure and constraints of data, for example. It's primarily an RDF serialization and the only benefit I see for AP is preventing property name clashes when doing full JSON-LD processing. I hope this topic will be reconsidered someday in ActivityPub 2.0.