From what did you conclude this? I read it as if it allowed login over serial for those accounts, he didn't mention ssh in that context.
Still, unprotected accounts seem like a pretty insecure idea to me. That's how our "attacker" got themselves in.
Of course in a free world physical access to a device you own would mean that you can modify its behavior ("hack" the software).
Sure, of course, but requiring a serial connection through a physical cable is still a less useful attack vector than ssh over wifi, which is what I gathered from the grandfather post - but not the article.
You are correct. I don't believe those accounts worked over ssh due to a lack of password, just local serial.