As TFA basically says: files on a filesystem is a DB. Just a very crude one. There aren't nice indexes for a variety of things. "Views" are not really there (arguably you can create different views with links but it's, once again, very crude). But it's definitely a DB, represented as a tree indeed as TFA mentions.

My life's data, including all the official stuff (bank statements, notary acts, statements made to the police [witness, etc.], insurance, property titels), all my coding projects, all the family pictures (not just the ones I took) and all the stuff I forgot, is in files, not in a dedicated DB. But these files are a definitely a database.

And because I don't want to deal with data corruption and even less want to deal with synching now corrupted data, many of my files contains, in their filename, a partial cryptographic checksum. E.g. "dsc239879879.jpg" becomes "dsc239789879-b3-6f338201b7.jpg" (meaning the Blake3 hash of that file has to begin with 6f338201b7 or the file is corrupted).

At any time, if I want to, I can import these in "real" dedicated DBs. For example I can pass my pictures as a read-only to "I'm Mich" (immich) and then query my pictures: "Find me all the pictures of Eliza" or "Find me all the pictures taken in 2016 on the french riviera".

But the real database of my all my life is and shall always be files on a filesystem.

With a "real" database, a backup can be as simple as a dump. With files backuping involve... Making sure you keep a proper version of all your files.

I'd say files are even more important than the filesystem: a backup on a BluRay disc or on an ext4-formatted SSD or on an exfat formatted SSD or on a tape... Doesn't matter: the files are the data.

A filesystem is the first "database" with these data: a crude one, with only simple queries. But a filesystem is definitely a database.

The main advantage of this very simple database is that as long as the data are accessible, you know your data is safe and can always use them to populate more advanced databases if needed.