not much detail, but slide 12 claims: https://www.platformsecuritysummit.com/2019/speaker/seay/PSE...

> Pluton validates and boots Security Monitor

> Security Monitor validates and boots the Linux Kernel

> Application Signatures are verified by SM and Pluton before Linux Kernel loads an application

This design still relies on prior stages of the boot process handing stuff over to Pluton - if there are vulnerabilities in the OEM firmware, they're still going to be exploitable in this model.