imo the best way to interpret those “real people don’t do x” writing advice posts is “most people don’t do x, so if a character does x, it should be a distinguishing trait.” human behavior is infinitely varied; for any x, there are real people who do x. we can’t make absolute statements. we can, however, make probabilistic ones.
for example, most people don’t address each other by name in the middle of a casual conversation. if all your characters do that, your dialogue will sound stilted and unnatural. but if just one character does that, then it tells us something about that character.
