> This backdoor only appears on decades-old VIA C3 embedded x86 processors

Modern Intel and AMD chips also have separate CPU cores that neither the user nor the installed OS control.

Intel Management Engine:

https://en.wikipedia.org/wiki/Intel_Management_Engine

AMD Platform Security Processor:

https://www.wikipedia.org/wiki/AMD_Platform_Security_Process...

Intel added them in 2008. AMD followed suit about five years later.