Since Intel/AMD also designs the processor they can also put in backdoors beyond ME, microcode updates, etc. If you don’t trust proprietary blobs, I respect that. But you can’t trust proprietary silicon either.
Yeah, microcode updates are proprietary software too. The weird result is that if you want a system with no proprietary software, you end up having to use the original microcode which is burned onto the chip and counts as hardware.
It's not a perfect solution but maybe it's a reasonable place to draw the line, until we have open source hardware processors using RISC-V or something.