I think more like this:
sudo mount /dev/sdb2 /mnt -o subvol=@
sudo mount /dev/sdb1 /mnt/boot/efi
sudo arch-chroot /mnt
I think more like this:
sudo mount /dev/sdb2 /mnt -o subvol=@
sudo mount /dev/sdb1 /mnt/boot/efi
sudo arch-chroot /mnt