I think it's someone who is actually extroverted, but do to being cast out as "weird", is now reluctant to talk to people despite their extraversion, and therefore thinks they're introverted.
This describes my case perfectly. I am extroverted, but due to people finding me weird, I was scared of talking to people, and mistook it for introversion. I am now way more outgoing than before once I figured it out, and people care a bit less about you being weird if you're likeable once you're older.
🫂