Since you’re using btrfs, there is likely another subfolder under /mnt. ll /mnt will tell you this, but the drive isn’t still mounted from the other day. When you’re mounting the EFI partition, you’re going to want to mount it to that folder, and not /mnt itself (/mnt/root/boot/efi, instead of /mnt/boot/efi) same for the binds (/dev, /proc, /run, etc)
Interesting, both of my F40 installs with btrfs only have a root folder, but it looks like yours has created separate ones for /, /home, and /boot. run
ll /mnt/boot; ll /mnt/home; ll /mnt/root
so I can take a quick look at where things are located. My best guess is that sda1 gets mounted to /mnt/boot, while everything else (/dev, /sys, etc) gets mounted to /mnt/root