Not all OSes have these same foundational pillars. What I meant to express is that Linux does whereas others (as disjoint as Genera, Windows NT, BeOS Amiga) don't.
Plan 9 was pretty much about taking the "everything is a file" thinking to its logical end point. Meaning that you could even manipulate individual GUI windows via the FS.
I was simply musing that Linux may have taken the concept further than the BSDs while still being "unix". I can't say i ever got the impression that Plan 9 was intended to be posix compatible for instance.