I work there, and I'd be massively surprised if it were.
Historically, their in-vehicle software has been handled by other companies they've contracted, and is radically different from generation to generation (different company each time).
It's not from what I read. MBUX, the UI is built using Qt.
That kind of implies that the new Mercedes OS will have some kind of POSIX API, since Qt doesn't really work without it. Their new Qt/MCU bullshit doesn't count.
Windows isn't POSIX, and Qt does just fine there.
Sorry, was talking more about embedded systems bindings. Qt has shims, obviously, to work with Mac/Win/GTK etc but when you're setting up Qt as the windowing system, it needs POSIX.
I'd be pretty sure that any company can pay money to Qt Company and have them come and integrate Qt into any kind of weirdo OS whatsover.