The Synology Drive version mirrors the filesystem, though I’m sure it has a database for sharing metadata. Is that what they mean?

I would say that basically all these software options use a database for things like preferences and user management.

Using a database isn’t some kind of heavy-handed horrendous thing depending on the implementation (e.g., as long as it leaves your content files alone).

Nextcloud too.

There is a database in most if not all useful cases, but there could also be the actual files separately.