I imagine there's a lot more going on behind the scenes that the author realises. If it's a single product startup a PM will be getting pulled in different directions by sales, marketing, tech, clients and investors. All who have their own agenda, all of whom may want different things.

It only sounds like PMs have all the power because they put the tasks in the order that you work on them in, after haggling with all the other stakeholders.

I'd guess that the PM in the story trying to get some evidence to backup a decision/convince someone of something.