My guess? He shared the Google Doc link with his peers, but forgot that Google Docs links are public for anyone who knows the link, so someone just forwarded it to oblivion, and he was forced to publish that as blog post. The addendums kind of reflect that.
That's a great reminded that any Google Doc with a shareable link is basically a public document for all intents and purposes.
There's not much difference between them ctrl+v-ing the link to a third party, vs them ctrl+a, ctrl+c, ctrl+v-ing the contents to another party. If anything, by just sharing a link, you have a chance to disable the sharing and hope the content hasn't yet been copied.
There is also a great reminder next to the button you click to get a shareable link in Google Docs.
As per design.
Google gives you sufficient control over access permissions. If you make it public, you have to know anyone anywhere can see it once they get the URL.
I frequently share Google docs and sheets links widely and to entirely unknown readers. That is part of the utility of the tool.
I read that as "for all indents and purposes"