arclanguage.org hosts the current version of Arc Lisp, including an old version of the forum, but HN has made a lot of changes locally that they won't disclose for business reasons.
The application is multi-threaded. But it runs over a green-thread language runtime, which maps everything to one OS thread.
That's a significant distinction because if you swap the underlying implementation then the same application should magically become multithreaded, which is exactly the plan.
There's an open source fork at https://github.com/arclanguage/anarki, but it doesn't have any direct relationship with HN.