What makes you think 17-year-old extroverts don’t feel the need to “fake it” to make it through any selective or evaluative process? The only ones who don’t are people who completely lack any insight about themselves, and who can’t tell the difference between their public persona and their inner core.
High-performing kids, extroverts and introverts, do lots of ECs, because our system encourages them to do lots of ECs. In Europe or Asia, the system doesn’t encourage either type of kid to do extensive ECs, and, guess what?, they don’t, extroverts and introverts alike. I’ll give you that extroverts here may do somewhat more ECs, but that’s because they are more responsive to external signals than the introverts. The quality of the introverts’ ECs may well be higher.
And no, I don’t believe there’s a correlation between studiousness and introversion, at least not in the Harvard admissions pool (or at least not in the pool of people who actually have a shot at getting admitted to Harvard).