in the bad olden times you definitely started to write your operating system for your new computer from the first instruction executed upwards. it wasn't at all common to have a comprehensive virtualization/simulation environment.
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?