That is not how you do it.
First you have to mount the root filesystem — in this case, the @ subvolume of the btrfs partition — to /mnt. Then, you must mount the EFI system partition to /mnt/boot/efi.
You’ve said this before, but I have no idea what that means. Is this — see the yellow arrow — what you mean? 
If so, that’s simply an indication of whether it’s mounted or not. In the above picture, it’s mounted.
Well, it’s a tutorial on how to recover from an interrupted update/upgrade. It’s not meant to deal with other scenarios.