You always "boot something that is unencrypted." You then "mount" the encrypted volumes and load the OS.
This is how people can put an SSH server (dropbear) in initramfs so they can unlock remotely.
The attack is to initramfs, not the encrypted layer.
The order'ish:
- Boot
- Initramfs loads, gives you the LUKS prompt
- Initramfs decrypts/mounts OS
- OS loads