That's baloney - virtually no computers these days are "cleanly separate" from their software, at least outside of tiny microcontrollers. Do you have any idea how much software executes before they even get to the point where the operating system beings to boot?
The idea that a CPU is going to come out of reset and start executing instructions that you provide at the reset vector is thirty years out of date.
Now that you mention it, I wonder what non-separable code would run on, say, a Corebooted Thinkpad running Linux with the Intel ME disabled. Surely that would be running 100% code that was not on it when it left the factory.
AFAIK, even fastidiously "liberated" core/libreboot Thinkpads like https://minifree.org/product/libreboot-t440p/ still end up using Intel's memory-reference-code for DRAM training etc.
