Hi,
Ok, I've added a setting in our source code repository if you want to disable the canonical url, so if you use the latest source code you can add this in user.config:
<add key="Forum:DisableThreadCanonicalUrl" value="true" />
Note that what you found in searching google for this site is correct because in this article google advises that if you have a multi page content then the canonical url should link to a full version on a single page. See mistake #1 in the linked article. Since forum threads have multiple pages we make it possible to flatten those pages into one page with all posts so that each forum thread is indexed as one document. In our paramter t=x~y x represents the thread id and y is the page number, if -1 is the page number then we show the full thread
Actually if someone is searching and the answer to their question does exist in the forum thread and they land at your site by clicking a link from google search then they should find what they are looking for. If they then have to click back and forth in pages of a forum thread to find it then it is not as good a user experience. According to google people generally prefer a single page when they are searching.
Hope that helps,
Joe