I think both are true to a degree. Introverts are often introverts because people or intense sensory situations wear them out. Over the last dozen years or so several people in my social circle found out they were introverts pretending to be extroverts. High social intelligence but no stamina for big gatherings.

I found a volunteer group that had a lot of ambition in an area I feel strongly about but hanging out with them too often wore me out. Amped up people amp me up. After some of their goals became reality the energy level dropped and the contact high wasn’t enough. So I show up at events and about a third of the meetings because the purpose is still there but it’s not enough.