All you do is poop all over the story about postgres. I'm convinced that no use cases will convince you of anything. I'm not really looking to involve myself in a database holy war.
Is jsonb in Postgres not flexible enough? I dump external json in there all the time (like large API responses). The jsonb operators work well. And there's an escape hatch that lets you easily convert json to a table. And importantly, you get indexes with Postgres.
Might I suggest you actually use mongo on a project before engaging further on the topic. It clearly has had a lot more thought put into it in the context of json documents.
If postgres works great for your use cases, great, go be happy with the tool.
Definitely a better solution than the brick wall he hit. Complicated beastie. I can see the appeal if you just love, eat, and breath SQL.
But a query language within a query language feels bolted onto something alien to do something pretty basic. Pass. If I'm teaching this to new engineers, pass also.