I am not confusing any such thing. You need to define "database" such that "file system" doesn't include it.

Common usage does this by convention, but that's just sloppy thinking and populist extentional definitining. I posit that any rigorous, thought-out, not overfit intentional definition of a database will, as a matter of course, also include file systems.

Perhaps so, but in the expansive definition you're using, even an in-memory binary tree qualifies as a database, which makes your point meaningless.

I'm OK including that — tmpfs is similar, but we can easily exclude that by requiring persistence. The intentional definition doesn't need to be expansive the point of being useless!

Nitpick: An in-memory hash table can be a filesystem :)