I'm not sure what this is offered as, other than one politician's choice of rhetoric.
https://taxjustice.net/press/millionaire-exodus-did-not-occur-study-reveals/
https://time.com/7377597/tax-the-rich-millionaires-wont-flee/
https://www.theguardian.com/world/2025/jul/10/friday-briefing-the-myth-of-the-fleeing-super-rich-and-the-real-tax-debate
https://www.peoplespolicyproject.org/2025/11/17/do-millionaire-surtaxes-lead-to-millionaire-exodus/
https://www.bigissue.com/news/social-justice/millionaire-exodus-myth-debunked/