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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.

Emphasis on the before.

AFAIK this is a completely new behaviour and only for newer versions of ME; the older versions still boot the main CPU like a 386 did, and the ME processor is a separate thing (Don't quote me on this; just information I gathered from brief research.)

yes true. the article deliberately focuses on reset-vector onwards steps. perhaps it should make that clear in the beginning. the ME mention is just to create a context for the starting point. of course ME is a huge topic in itself