Not saying there couldn't be a targeted supply chain attack (that's essentially what was revealed in some of the Snowden leaks, e.g. targeting networking cables leased by big tech companies), but I don't believe there is some widely dispersed secret backdoor, even if just for the reason that it's too hard to keep secret.
Any analysis I could or should do?
Interesting, how would an X86 instruction with hardcoded 256-bit key would be detected? IIRC it's really hard to audit the instruction space for CISC architecture.
But I cannot believe they resisted the temptation to use that opportunity to get such an easy access to so many devices.
Consequently, if there is an ME-subversion, it's only deployed / part-replaced for extraordinary targets. Not "every system."
But it has been a while that I read about it and I never took it apart myself, so maybe what I wrote is not possible for technical reasons.
Otherwise, you need something in your OS to ship data back and forth between the ME and whatever NIC you have.
This seems like as good a short-form intro as any: https://blogs.cisco.com/learning/security-in-network-design-...
But when targeting a random individual in a hurry, I think it would be handy to just use the build in backdoor.