It's also right there in the "common sense theory"

The line between "an advanced, niche concept" and "common sense" is very blurry. It's continuous, not discrete. What you think it common sense is probably not for a considerable proportion of other people.

Regarding the actual topic at hand though, I actually do agree with you (hence disagree with OP), since I do believe that mastering concepts like color theory and typography are indeed extremely low on the extensive priority list of things to master to create great UIs.

I've been building them, and convinced people to pay me quite handsomely for them, for quite a considerable amount of time, and never have I had to deep-dive into typography, color theory, or the like.

Well, color theory is a vast topic, especially when it comes to color reproduction and so on, but for UI design this is of lesser concern. However, basic stuff like the color wheel, complementary colors, perception of colors in terms of perceived contrast, "signaling" colors, knowledge of optical illusions etc. is pretty much 101 knowledge of anyone who wants to create really good designs. There are many traps you can fall into if you ignore this kind of knowledge. This goes even further with the entire field of cognitive psychology dealing with such things.

Typography also plays parts in human facing design, like being able to discern I and l and other such things. You may get by without all that, but I have seen too many cases of broken stuff, and that website from the OP is a big mess, pretty much counterexample of what I call "good design".

Crossing into "visitors can't read it if they can't see it theory".

The contrast ratio for the body text is 7.57:1 (or 7.25:1 if the text is on the light grey background), which meets AAA accessibility standards (which is 7:1).

Body text isn't the issue I would think - it's various headers, downvoted and especially "dead" posts which are almost to read without selecting the text, even with perfect colour vision.

(oops, missing word 'impossible' after 'almost' there)

I can tell you that it still looks rather low-contrast and makes it harder to read for me.

This is also in part because of human perception, i.e. it wouldn't be as bad if the rest of the page had the same level of gray for the text. Because it is not the case the darker parts suck your focus in. If you want people to actually read your text, that is a pretty stupid move and just shows how serious you can take all that spam on this page.

Why would I care about that standard? I have my own eyes, I also see a lot of other examples of text that don't race to the standard bottom

Also, you've miss the elephant at the top of the page

The only UI/UX newsletter I subscribe to[0] also has a a similar problem, but the advice is usually solid. Maybe the first and/or last rule should be ‘seek feedback’.

[0] https://user-interface.io/