Thanks Justin!

The specific reference isn't about professional traders losing money, it's about professional traders beating the market (ie - S&P 500).

I first learned about it in business school and there is a lot of evidence for it. Here's one example of many...

https://www.ngpf.org/blog/question-of-the-day/question-of-the-day-over-a-recent-20-year-period-what-percent-of-pros-investing-in-large-companies-beat-the-market/#:~:text=Question%20of%20the%20Day%3A%20Over,)%2C%20so%206%25%20outperformed.