Even includes 2 videos for those who aren't too familiar with pc insides.
It looks like they are using parts that are widely recommended here. So it brings me to think that someone at extreme tech checked this place.
It would hardly be surprising that they know of the SPCR website. Aside from just googling "silent pc" or "quiet pc", there was a front page article about SPCR in the Wall Street Journal several years ago, and a couple of the members of this forum where featured. Or you can check out the wikipedia article on Quiet PC, which is obviously written by a member of this forum.