The security of any operating system (OS) layer depends on the security of every layer below it. If the CPU can’t be trusted to execute code correctly, there’s no way to run secure software on that CPU. If the bootloader has been tampered with, you cannot trust the kernel that the bootloader boots. Secure Boot allows the firmware to validate a bootloader before executing it, but if the firmware itself has been backdoored, you have no way to verify that Secure Boot functioned correctly.
This problem seems insurmountable: You can only trust the OS to verify that the firmware is untampered with if the firmware itself has not been tampered with. How can you verify the state of a system without having to trust it first?