I am not a chatty person - crippling social anxiety sees to that - and I would much prefer this kind of informal chat to a "real" interview because it removes a lot of the pressure which then helps to alleviate the anxiety enough to show that I know what I'm talking about.
If you can't express your competency during an interview chat I don't see how you could be a good candidate (there are exceptions), nothing more damaging to a team that someone who can't communicate.
How about shy people who have a hard time talking to new people? Who only feel comfortable after a couple of days and then they become very chatty? Aren't most developers like that?
Not significantly more than among the population in general, at least not in my experience.
Being able to simulate extroversion for short periods at need is a useful life skill for any introvert - I've found it invaluable in plenty of situations beyond just job interviews.
No necessarily competent. It could be that they are well read on the subject but have no practical experience in it. So they seem able to converse in the subject's vocabulary but if they were to sit down and try to make something they would fail.
