No, it's real. It used to be prominently on the site sidebar, but it got very big around the time xkcd first got popular, and the link was subsequently removed. After that, you had to know it existed and just go directly to forums.xkcd.com, so the only people who knew of it were generally the people who were early xkcd readers or people invited by them. Imho it's one of the better open "offtopic" discussion forums on the web, partly because of insular culture trending towards thoughtfulness.