> Even in the context of written mathematics, this is a very unusual style. I feel like J fans talk about it as if it's a totally normal thing to do if only you knew a little more math.

Yes, it's very unusual. I think the argument is more that there is a tradeoff, that "inflation" (whitespace, long names, multiple classes/files/etc) has a cognitive cost too even it it's more approachable, and that this other alien-seeming style can work very well for some people. There's a lot to love in the APLs even you ultimately don't buy the fanboy arguments, if only for the lessons of just how different the world can be.