Lots of those details are vendor-specific. Really the specifified behavior exists in the modern world not to enable "boot" (which as you point out is mostly the job of firmware running on the ME and not the application CPUs), but to clearly define the initialization sequence for SMP startup (i.e. how do the auxilliary CPUs start -- Yes, that's right, they start in real mode) and some other minutiae like mode switching for legacy interaction with BIOS calls.