Making the team stronger often doesn't make your career better though. This guy says he didn't and he got promoted to manager anyway, and now suddenly he wants the people under him to make his team stronger rather than focus on their career like he did. That smells like a corporate climber who just started to eye his next promotion rather than a new insight.

Edit: Notice I'm not saying this is bad, this is just natural bias towards what benefits yourself. It is just so interesting to see how people who experience that bias shift now sees it as some kind of "insight" rather than just their own bias.