Let me rephrase.

We already have a bootloader.

"Linux from scratch" involves Linux, not Libreboot, not Grub (just to mention instances of BIOS/UEFI and bootloader).

We already have means to test the new OS.

What I meant with my previous post is "newer" bootloader (which are actually pretty old, like grub), came after OSes passed a certain complexity.

Previously we technically had bootloaders, but they were interactive, mostly.

Am I misremembering?

oh. I'm talking about authoring OS and boot loaders from an absolute blank slate. like starting from the world where you toggled them in from switches, and later by burning proms