People do tend to think this way, but when applied to the topic that the forum is built around, this line of thinking is probably accurate.

People on HN are probably more tech-savvy than the general population. People on DefensiveCarry are probably more CCW-savvy than the general population. People on /r/Seinfeld probably tend to be some of the show's/comedian's biggest fans.

I understand and agree that the claim that we have more "complex personalities" is a little suspect, but a lot of these sorts of claims probably have something to them.