That’s 1.7 million webpages using that exact phrase, that’s probably 1.7 million hidden backlinks, some serious link benefit they are stealing from their forum software users! There’s an SEO market for buying/selling links, it could cost hundreds of thousands of dollars to buy this number of text links from quality websites and they are getting them for free.
If a webmaster notices the above links in the Googlecache say and check out their forum (viewing source) they find the link within the noscript tag, which though not a particularly ethical way to gain a link it does at first viewing match the Googlecache so doesn’t look like cloaking.
In my opinion this is a deliberate blackhat SEO attempt to hide the backlinks from forum owners!
In comparison phpBB forum software has a similar powered by link, but it’s visible to everyone (forum visitors and Google) and there’s loads of articles online about how to remove the phpBB forum links.
By hiding the Discourse.org links this way only the most SEO knowledgeable webmasters are going to notice these text links and look to remove them.