alt Hacker NewsI believe the original statement is an oversimplification. What actually happens is that extreme situations, both positive and negative, can help you discover things about you that you didn't know before.
Apart from that, the problem with "who you really are" is that individual is more of a process than a static thing, so any such reification becomes invalid in the next instant.
You're right that people aren't static, but we should also acknowledge there are lots of people who become rich and powerful and they don't do horrid things. Many are perfectly decent people who care for their families, help those around them, contribute to their communities and use their wealth and power to support causes that are important to them.
You don't hear about these people as much because they're not out looking for attention, making outlandish statements or even trying to "change the world" in a narcisstic Silicon Valley way.
"Who you are" at your core drives the direction you go in when you acquire wealth and power.