A platform for amateur XNU kernel research was required, since the only other one besides the Mac is the Security Research iPhone (which is specifically not for amateurs). All of Apple’s documentation (of which there is little) and the boot loader design itself (what with the blessing process for a secondary drive being to install a tiny XNU partition with the bootloader app), point to the process being specifically designed for loading different versions of macOS. Any ability to install Linux (or Windows) is at this point coincidental.