Brunch changed the way the EFI firmware is handled within the kernel, and `/sys/firmware/efi` is now called `/sys/firmware/brunchefi`. This guide relies on the fact that the mokutil provided in the Debian repos look into the `.../efi/` directory and not the latter. If you want to proceed with this guide, you have to compile mokutil and its dependencies with the new path yourself.
ChromeOS is something so efficient yet sometimes so locked down OS and it's what I love the most about it. It provides you flexibility while locking down things that only researchers and power users would be concerned about.
Right as I downgraded my ChromeOS system and Brunch Framework back to stable releases and doing some random research about ChromeOS (I forgot what specifically), I came across this Google support article explaining the differences between regular ChromeOS and ChromeOS Flex, out of which this specific point caught my interest: