I've always found it kind of ironic (or at least funny) that the code for HN isn't open source.
Not sure that’s always been true, or at least for sure PG was open about subsets if the code. Generally, there’s reason to open source code, and open sourcing it would likely result in more overhead not less; generally HN goal is to very, very slowly change the code, interface, etc.
Honestly, cloning HN really would not be that hard, cloning dang, that’s another story.
EDIT: Here’s a recent comment from dang on open source HN:
There is an open source fork but it runs on Racket[0], so the Arc documentation isn't always correct, and of course that forum and this one differ in terms of features.