Mount the true Btrfs root somewhere else so that you can access its contents. The Btrfs's true root is known as subvolume ID 0 (and is not listed by btrfs subvolume list). Find or create a free mount point and mount as follows:
mount -o subvolid=0 /dev/some-device /mnt
Now you should be able to see /mnt/@, /mnt/@badroot, /mnt/@home, plus maybe some other files under /mnt. You can delete subvolume /mnt/@badroot as usual with
sudo btrfs subvolume delete ...
Credits: https://unix.stackexchange.com/questions/188860/how-to-delete-btrfs-subvolume