@encore Remember these words:
Code equals bugs therefore fewer lines of code will lead to fewer bugs.
Coreboot uses a more minimalist design philosophy (for example it doesn't overly rely on SMM and other nasty things). A lot of these firmware-based attacks target edk2 which is the reference UEFI implementation from Intel.
Libreboot actually does provide UEFI now, but implemented by U-Boot instead of edk2. U-Boot has much higher code quality and (IMO) better auditing than anything edk2-based.