I started from a new (previously unused) USB flash drive and followed along the instructions until
sudo mount /dev/sdb1 kernel, which gave me the following error:
mount: /home/xxx/Downloads/openwrt_edgerouter_lite/kernel: wrong fs type, bad option, bad superblock on /dev/sdb1, missing codepage or helper program, or other error.
I fixed it by creating a file system in the first partition:
sudo mkfs.fat -F 32 /dev/sdb1
Should the instructions perhaps mention this somewhere?