Like in US supreme Court "I know it when I see it", definition isn't straight forward but it has elements of "is it depiction of a sexual act or simply nudity ", as well as any artistic quality. Generally, erotica has high production values and porn less so.

Anyhoo! What a weird place for discussion to end up :-). The story is excellent and very hacker news appropriate, but his entire opus is pretty good. There's a bit of deus ex machine in some of qntm's work, but generally they have the right mix of surreal and puzzling and cryptic and interesting to engage a computer geek's mind :-).