It was a great saying he is the CEO of a product he is making. That was a truth for a while. Not anymore.
In most of the teams right now PM has no leverage except his expert weight. It's while trying to find a balance between business, development, and user happiness. So who we are, what we do, and what is our role?
We are the ones who collect knowledge about industry, development, user, and stakeholder expectation. Ones who know how to find information via various kind of research (market, UX any means needed)
We provide this accumulated knowledge to the dev team as documents and answers.
It's not our role to rule. Our purpose is to lead the knowledge provided. What we don't know and what experiments we need to do to know more? What are the numbers and best practices? By being an expert, you take the cognitive load from fellow team members. And if you are knowledgeable and can defend your arguments, the team will support you.
(hint: both developers and business are good with numbers)