Disagree, IBM i uses files: how do you create a database table in it? CRTPF command ("Create Physical File"). You create a file.

And I don't know what you mean by "catalogs", in the context of IBM i. Are you talking about DB2 catalog views? (Which exist in DB2 on every other platform, and most other RDBMS have something equivalent, such as the ANSI standard INFORMATION_SCHEMA)

Or are you confusing IBM i with MVS (in which the OS contains databases called "catalogs", in which you lookup a file name or file name prefix to find out which disk volume a file is stored on?)

I might have mixed it up with object libraries.

Makes sense. Although, what's the difference between a "library" and a "directory", other than the name? I think the difference is more terminological than conceptual.

There's a cool little demo [0] of AS/400's files-as-databases concept:

[0] https://www.youtube.com/watch?v=CDSgJE5mPJM