The original version was open sourced (Perl artistic License) http://arclanguage.org/ There is an active fork in https://github.com/arclanguage/anarki but it's totally independent and the current conde in HN can be (very) different.

My guess is that it's very difficult to keep all the details of the secret sauce hidden. They change the details very often. For example the front page is ordered by points/time^1.6, but the 1.6 changes from time to time without notice (I think it was 1.8 for some time, perhaps it's 1.8 or something else now. Some people have analyzed the front page and got compatible results, but I don't remember the exponent they found and I'm too lazy to try).