Earlier quoted context omitted.
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.
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.