I'm not saying this was malicious, but "debug version w/ backdoor added to prod by accident" is a great way to explain away a driver literally named SecureBackDoor. https:://www.welivesecurity.com/2022/04/19/when-secure-isnt-secure-uefi-vulnerabilities-lenovo-consumer-laptops/
I'm skeptical of backdoor hardware implants en masse not because they're impossible, but because backdoors in proprietary firmware are easier to implement, to hide, and to explain away when discovered.