This is a cool exercise, but I would hesitate to choose files over SQLite or another Dockerised relational database in production.

They are overoptimising for the simplest part of writing the application; the beginning. They've half-implemented an actual database, with none of the safety features. There are a lot of potential headaches that this article has avoided talking about; perhaps because they haven't experienced them yet.

See: https://danluu.com/file-consistency/

What happens when you need to start expanding the scope of this feature? Joining users on profiles, or users on orgs?

Ask yourself: how many shops have seriously written an application backed by files and stuck with it over the long-run? The answer is likely very few. Therefore, this is likely doubling up the work required.

There is a reason people reach for a database first. I'd strongly encourage anyone to avoid doing stuff like this.