If I'm setting this up during the day, when I normally work, those external sites would easily end up with the "auto" behavior by this very poor design in the blog, when that is not my actual intent. This would result in me having to set it twice (the second time in the evening) to have my desired behavior which is a nuisance. The explicit option is better here, not whatever this silliness is in the article. [0]
At least I've read this article so sites and apps using this stupid option wouldn't be too big of a surprise to me, I'd hate to have to explain this to other people who haven't read it though. Principle of least surprise is a good thing, not something to deliberately work against like this author wants.
EDIT:
[0] Actually, this demonstrates a crucial flaw in the system's design. If I want to set the current default as the permanent setting, I have to wait until a particular time of day to do so. That's even worse than I'd previously realized. You can only set permanent light in the evening (or by changing your OS/browser settings, changing the site, and then changing OS/browser back). You can only set permanent dark during the day (without that same stupid exercise).
This is a terrible design concept.
There is no way any one with any common sense can think that this two-choice option (one is magic, one does what you would expect; which is which depends on time of day for many users) is better than a three-choice option where each choice does exactly what you'd expect.
In my experience, software that is designed to work only for the most average user ends up feeling terrible for most everybody, because most people deviate from the average in one way or another, at one time or another.
It feels especially bad when the software makes assumptions about what the user wants, to the point of including a button that doesn't always do the same thing when pressed, and which doesn't even tell the user it doesn't always do the same thing when pressed.
Just look at all the complaints Microsoft gets for the way Windows constantly reverts or ignores user settings.
Or the way it feels to google a very specific set of search terms, only to receive back 20 completely irrelevant listicles that answer a statistically more likely question you didn't ask.