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.