They use a criteria to determine it. It mostly consists of many types of failed interaction with other people over the course of many years and an inability to deal with it easily like most would. Kind of like how a soldier might have trouble adjusting to being a civilian after many years of fighting in a war, although war trauma is different, but the element of failure, loneliness, and fear is in its basic forms pretty much the same. Some are pedantic, some are not. Maybe because they learned they get less shit if they shut the *&^* up and just talk to themselves in their heads. Some say they are smart, some say they aren't smart. Some have learned enough how to act normal that nobody would assume that in their heads they think "I hate talking to everyone" because they know they have to pretend and it's very tiring not being able to explain why you are uncomfortable to someone who doesn't have the slightest clue and will take it personally because of the huge difference.
I'd like to know the ratio of how smart a human is to how much bullshit they can get people to believe. You know, kind of like how some 'smart genius scientist' (whatever that means) comes up with a cure for 'depression or all different types of mental illnesses' (whatever that means) and then when someone shows evidence of that being false, pulls a 180 and says "Oh by the way, this is still a new science. We thought we had it, but we were wrong." That's some good stuff. Nice to know there are always people who think 'good intentions and present sacrifices' justify doing more harm than good to a person.
Outside of MBTI as a loose theory to help individuals interact with those they know very little about, this makes me hate psychology. It's all complete conjecture. Ask multiple types of people from different ethnicities and countries what their problems are and how they deal with them and what they think of them and there is always a different answer. Psychology should be looked at with great suspicion.
I know TL;DR, wahhh wahhh wahhh Charlie Brown.