Nosql is a fun target to beat up on of late. But there are good, even infamous, reasons to avoid SQL. Particular if you want to accomplish flexible record queries from untrusted clients.
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.