Here is how they actually set the kernel for booting. It's hidden behind a `curl <url> | sh` in their post:

    #!/bin/sh
    bputil -d | grep "CustomerKC" | grep -v "absent"
    KC=$?
    if [ $KC -eq 1 ]
    then
      bputil -n -k -c -a -s
      csrutil disable
      csrutil authenticated-root disable
    fi
    cd /Volumes/Preboot
    curl https://$LONGURL/linux.macho > linux.macho
    kmutil configure-boot -c linux.macho -v /Volumes/Macintosh\ HD/
    echo "Kernel installed. Please reboot";

So, it looks like they download their pre-compiled kernel onto a Preboot volume. This is then set as the booting kernel? How long until there is a grub-like option?

I should add -- I don't know what most of these commands are. It would probably be helpful if they had spelled these out more, but this is all a great place to start.

bputil - Utility to precisely modify the security settings on Apple Silicon Macs.

csrutil - Configure System Integrity Protection (SIP)

kmutil - replaces kextload, kextunload, and other earlier tools for loading and managing kexts