I think you're missing the point of the comment you've replied to. That comment is talking about implementing the DB. When you're implementing a DB, you can't just forward reads and writes to another DB. Someone has to actually implement that DB on top of a filesystem, raw disk, or similar storage.
With the post's logic if you work on DB you likely have an SQL engine that is a CRUD on top of storage engine. And storage engine is a CRUD on top of some disk storage, which is CRUD over syscalls on files. Think Mysql using MyRocks using RocksDB.
And keep applying until there's no sense left.
If you are referring to my post, it's about web applications. I'm not in any way claiming that postgres is a crud app. I'm describing how to design a good web application that mostly revolves around a database. Which is what people mean when they say crud app. It's just any app that's mostly crud. Where the majority of the logic can be handled by the database like I described.
A lot of apps are just about putting data into a DB and then providing access to that data. Maybe using the data to draw some nice graphs and stuff. That's a crud app.
