Yes, I'm using MongoDB API in Pongo to keep the muscle memory. So, it's a library that translates the MongoDB syntax to native PostgreSQL JSONB queries.
Yes to "The posted project looks like a client that connects to pg but behaves like Mongo, where Ferret is a server that accepts Mongo client connections and uses pg as backend storage."
Yeah, FYI that's a sibling comment, not the one you replied to.