I have a longstanding fascination with K and other "modern" APL derivatives.
There are a few intersecting truisms about coding that I believe: one is that people's working memory varies: some have an immense amount, some less. Humans definitely process spatially better than in time series (e.g. comparing side by side rather than turning over a page.)
This implies you should prefer succinct code and languages because they are less memory load for engineers working on them.
At the same time, a corollary is that a smaller standard library / language is generally better, in that less needs to be learned by an engineer for full coverage of the language.
Another truism is that some people's processing speed is higher than others, and in general I think of the combination of working memory + speed as roughly equivalent to "g", general intelligence.
K occupies this weirdo place though, because it's absolutely succinct, a very small language as counted by number of atoms supported by the interpreter, and also incredibly hard to scan.
One of the K intros I read mentioned that the language is designed to be something that takes down your thinking; essentially the idea is that the workflow is "drink coffee with fellow PhDs, annotate on the chalkboard, and then when ready, capture it directly." This seems about right to me with my own K/J/Q experiences -- the bulk of the time is spent thinking about structuring a problem solution.
I compare this to go, a language I love for its long-term readability and maintainability, where I spend a lot of time writing boilerplate and dealing with errors in-situ.
At any rate, somehow there's a sort of event horizon of terse solution making where you come out the other side and need a 170 IQ to feel comfortable, and Mr. Whitney lives where he lives, and I live where I live. :)
People complaining about how ugly the C code is here are definitely missing the point: he has bent C's preprocessor to his will in order to encapsulate how he thinks about coding: essentially functional, vectorized. It's using C to write a DSL for solving programming problems interesting to Arthur Whitney.
I think it's fascinating on those terms. In a world where you have to read 10,000 lines of code from 100 developers, the C is terrible, and hard to parse. In a world where you will mostly write code to a style you've honed over 40+ years, it's super expressive, minimal, pared down to what matters, and probably fits his brain perfectly.