Your colleague is sorting people without really being aware of whether he's accurately sorting them—for example, how would he identify non-Mormons that didn't have extreme tatoos, piercings, and a need to constantly prove that they're non-Mormon?

Since I've left the Mormon church, very little has changed about me. I still have a short haircut; I still dress conservatively; I have no piercings or tatoos; my language hasn't changed. And because of all this, I am, on a regular basis, grouped with the "Mormons" in spite of the fact that I am very much not Mormon—my coworkers address me as a Mormon/use Mormon in-group language with me almost every day, and I don't correct them because it's not a big deal.

At the end of the day, this is much like a thread where people congratulate themselves on being able to spot liars, oblivious to the fact that they are poor judges because they are unaware of all the lies that slip past them undetected... Nothing more or less than basic confirmation bias.