It's a red flag to assume that I'm going to want a raise, regularly? I don't know that it's my undervaluing of the work that's the problem ... I'm fairly certain that the company undervaluing my work is the problem that makes me leave.

The joke was that your not undervaluing your work is the problem, because your employer would like to underpay you :)

They're saying the fact that you have made this realisation and are prepared to act on it is a red flag to employers.