Right. If you had followed the guide, you would’ve noticed that nowhere does it talk about using a client to connect and perform the various commands. You need to access it with a keyboard attached anyway to do the initial configuration before you can connect remotely, so rather than introduce another variable (having someone install something else), I opted not to do that.
That said, I’ll make a note in the guide. When connecting remotely, you must run the command as:
$ DISPLAY=:0 npm start