Nothing. I tell them absolutely nothing. I behave as if I didn't hear them, or at most say something like "that's interesting!"
I avoid telling people they're "not introverts" anymore because even if they really are extroverts, mental illness can make it complicated. If they have something like bipolar disorder that makes them physically incapable of shutting up when they are in mania or hypomania they are probably very self-conscious about talking too much.
Who is to say in a case like that whether their "true self" is an introvert or an extrovert? Mental illness can make it messy.
It is the same for extroverts who have social anxiety disorder, or problems with shyness.
When it comes to T and F and J and P it's a little more clear-cut. N is pretty clear too but it's so important I don't talk about it, because the risk if I'm wrong is too high.
People also go through different phases, they hone in on different functions, and if they make a claim without asking my opinion it's probably because the conclusion they're already at has some value to them at that time.
This was something that I had serious lack of understanding of as a teenager; honesty and accuracy when speaking literally has always mattered at lot to me and it still does. However, in many situations the value of someone having an accurate understanding of something as arbitrary as a personality type can be outweighed by the way volunteering an unnecessary contradiction could make them feel.
So to anyone who has told me that you are an introvert who reads this, I hope it doesn't leave you feeling too unsettled. But if it does and you want to get mad at me I can accept that, but I hope you tell me so we can talk it out, and that you will give me the chance to reassure you that neither being introverted nor extroverted is a bad thing, and why what I think about this doesn't really matter at all.