There is no definition of what an MPU is and why this is a different "community" than the MCU with its sub communities. Please explain. You might put that in the description section of the forum two forums.

It means "Microprocessor unit", because it has no RAM and flash in it. Usually those are with Cortex-A core(s) not Cortex-M, and often is called also SoC (system on a chip).

The MPU boards are designed to run Embedded Linux properly, so you're building kernels, writing drivers, and apps running on the OS. I suspect they kept the things separate so as not to confuse the chaps using the MCU class devices, and those claiming to "know linux" yet have never built a kernel.

The bigger question was what fool thought calling them STM32 parts would help separate functional differences and expectations.

For me it is super obvious. It is not hard to guess what MPU means when the word is used ​alongside with MCU. It's a microprocessor that is capable of running normal Linux. And it definetely need a separate communities because it's....Linux.