To remove the BBL from the devices that make up the array that the root filesystem is on you would need to boot into a rescue environment and assemble it there.

To remove the BBL from the devices that make up the array that the root filesystem is on you would need to boot into a rescue environment and assemble it there. While the debian-installer does provide the rescue environment that you need to do this, it is by no means trivial. Speaking as the article author I find it way way simpler to create the arrays from the shell at the start, rather than have to remember to reboot into d-i again, choose the rescue mode, tell it not to automatically assemble, drop to shell and assemble there. It's also faster — most of my servers take more than 60 seconds before even the serial-over-lan starts to display output!