Arc is just the language the forum was written in yeah? Is the forum system itself open source?

Yes, the forum is open source too.

Although as far as I know the only thing implemented in Arc is the forum so they might as well be the same thing.

Both the language and the forum are licensed under the "Perl Foundations's Artistic License 2.0."

If you download Arc, there is a text file called "how-to-run-news" in the tar.