Then the 8-bit ATMega328p in the Arduino Uno is an MPU[1]. Except it's solidly an MCU. I'd say if it uses external RAM as its main memory, and external storage as its main storage, then it's an MPU. If the RAM is all internal, and the storage is all (or can be all) internal, it's an MCU.
That’s a gimmick. I’m talking about how given chip is supposed to be used in a real products, whether vendor supplies Linux builds in one way or another, etc.
[1] https://github.com/raspiduino/arv32-opt