…AMD too with their counterpart…

AMD’s counterpart PSP doesn’t seem capable of what you’re suggesting. As for Intel, well, they don’t have the same comfortable lead anymore so I strongly doubt they’re going to try anything like that, making it a moot point.

Given that, do you think that all those millions of fully secret lines of code cannot render you unable to use your PC, even if just temporarily?

I never said that, so I don’t know what you’re arguing against. Of course they can remotely disable an install of Windows. But being able to brick a device, rendering it a mere paperweight forevermore? That is what I am saying is not possible, and I’m fairly sure I’m right about that.

As for the UEFI shenanigans, well that’s a feature of UEFI, not a bug. However there is a level of trust you’re supposed to be able to have in your OS not to mess with it, and is just a sign of Microsoft’s arrogance that they do.