The article definitely oversimplifies the IO happening in a database.

That said, depending who you are talking to, they may not agree with you on "mmap is much better than this". Some people will say you should do what you need in the application logic instead of depending on APIs from the OS. (although not necessarily for the specific example here)

https://db.cs.cmu.edu/mmap-cidr2022/

The backing store being used by map() is still a file in a filesystem, so I would say their overall claim is technically true. It's the "exactly like your code does when it calls open()" part that oversimplifies a little (though, again, remains technically true -- it's just giving an example of a thing you can do with a file, not exhaustively listing all the things you can do with a file).