To be perfectly frank, I did not find this article informative, and I would not consider myself a vim expert by any stretch of the imagination. I suppose it was aimed at beginners, so maybe some people will find it helpful.
You're right, it's not aimed at advanced or even intermediate users. To be honest I'm surprised (though flattered as I usually am) that it got onto HN. Hopefully it's useful to at least some people to clarify Vim's behaviour, since it may not be familiar to people raised on GUI editors like myself.
I also found it useful. I'm just coming back to editors, specifically Vim, after years in IDE-land on Windows, and your simple definitions provided a nice mental model for me to build on.
See also `:help windows-intro`.