root must always be mounted if you have a system, either by booting or chroot. If you mount just efi from another system .
On my setup /efi/EFI/grub only has the grub efi binary no config, /boot and therefore /boot/grub is on /root partition
Now if you have the entire /boot in the /efi partition then it would be /efi/boot/grub/grub.cfg?