I'm no psychologist, but I think it's a matter of the degree/extremity of the behaviours described there.
Withdrawal is a real issue but the description is problematic, especially the use of the word preference. What I think they are trying to say is an active preference to socially detach from other people. We introverts don't really have a choice but those who are not have, and if they choose to over-ride their natural tendencies and withdraw, then it could be a symptom of a problem.
In other words, this is what happens when a non-introvert starts acting like one