I thought Unix was “everything is a file” and Plan9 was “everything is a filesystem”?

Maybe so. I guess the point is, the concept that different types of resources need different protocols is baked in, rather than picking one type of abstraction and applying it to every type of resource.

https://doc.redox-os.org/book/ch04-10-everything-is-a-url.ht...

There are some filesystem-like usage patterns built on top of that which are universal, but they're more limited.

https://doc.redox-os.org/book/ch04-06-schemes.html

On Unix "everything is a file" until sockets came into the party.