This must be an acronym mixup. We’re talking about the Media Control Unit, i.e. the giant screen in the center of the dashboard responsible for zero safety-critical systems.

Yes, The infotainment stuff, like the screen, internal lights, speakers are connected via a low profile third bus system, certainly not the main CAN bus or profibus or firewire. Some, like in the BMW even via WiFi. These are the systems usually used with Linux or Windows or Android. On top of the important stuff.

MCU in the case of braking and steering is “micro controller unit”.

Yeah, those definitely aren’t running Linux, and the guy here wasn’t working on them.

Didn't they already do an Over-the-Air update to fix the braking time in response to a bad review?

You don’t need an OS for that.