My terse summary of this post's argument is "I have good aesthetic sense and unremarkable calculation ability; I was able to translate my aesthetic sense into mathematical ability. Scott Alexander has great aesthetic sense, thus he should be able to translate that into mathematical ability, even in the presence of poor calculation ability."

I think this only works if you keep 'mathematics' a broad and vague category. Yes, Scott could probably do well in a group theory class--when I took it, I was surprised at how much of it would have been intelligible to a much younger me, whereas other math classes I took at around that time really did require linear algebra and calculus and so on as a foundation.

But being good at group theory is different from being good at "math" in general! If Scott's dream was to become an actuary or accountant, aesthetic sense would be irrelevant in the face of calculation ability. If you start with the goal (say, mastery of Bayesian probabilistic reasoning, or causality discovery, or so on) and then try to learn the math necessary to achieve that goal, it seems obvious to me that someone could be poorly matched to their goal, and possibly unable to succeed even with the expenditure of heroic effort.

(If one starts with one's abilities, and then finds the goal that is best matched to those abilities, one can obtain much greater success--but may value that success less.)

It's just not true that if someone has substantially more trouble learning scales and chords than his or her classmates, he or she is "worse than them at music."

So, I'll defer to Scott if he disagrees, but my impression is that he has substantially more trouble learning scales and chords than his brother, and that he is "worse than him at music." It might not be logically necessary, but we can certainly notice that it is probabilistically likely.