Despite what the maker-movement and IoT-hype would have you believe, most embedded systems are not running Linux.
Bear in mind that the vast majority of processors sold these days are micro-controllers and go into things like electricity meters, light switches, alarm clocks, etc.... mundane stuff.
There's benefit for companies to do that too, developers who can work efficiently on an MMU-less bare metal/small RTOS system tend to be experienced and cost a lot. Now if you run Linux (or maybe even android these days) on your embedded product you can basically have any half competent junior coder develop on it without problem.
At the rate this is going I would only be half surprised if my electricity meter ran Linux in a decade, especially since the trend is into making "smarter" ones.
Also while Cortex-M are definitely more microcontroller-ish than the A versions they're still pretty impressive beasts compared to the microcontrollers of yore. They can run at frequencies on the order of hundreds of MHz, often come with advanced IPs like USB, ethernet, DMA and more. Maybe we should have an intermediate category for "millicontrollers".
And maybe it should. I think as far as software is concerned embedded development is not all that special anymore. Everything is running advanced ARM SoCs that have the same power you'd have on a workstation ~15 years ago.
It's still meaningful if you're working on electronics though, you generally have wildly different constraints regarding ECM, power draw, heat dissipation, environmental constraints etc...