EFI. I started using SB on said computer when Nvidia support got added so a late 29 build or beta 30 build.
This too was in my config. If it’s any help, I did have osprober disabled as it prevents ostree-finalize-staged from taking forever on systems that have a Bitlockered Windows on another drive.
Instead of using /mnt as a chroot directory, use /mnt/ostree/deploy/fedora/deploy/<deploy id>. You’ll need to mount the dev, proc, /boot etc there too.
rpm-ostree won’t work, but grub-install does.
I thought my computer was already using BLS, based on the /boot/loader/entries