IME Does not have a separate power source.
Obviously backdoors like this are bullshit but you have a tendency to exaggerate the situation to the fringe.
The only cure to such fuckery is an open-source decentralized modular solution to hardware, but unfortunately that is a wildly complex and tall order. Maybe one day.
OSH is the solution to the KYC internet that is coming, too. Without OSH we will only be allowed such information and ability to communicate as them we depend on for our devices and network infrastructure want us to have. Also, it's not only Intel, but all commercially available chipsets. Moving to AMD won't end the hardware backdoor.
Potato Potahto. You can't turn it off, and turning off the machine doesn't turn off the IME.