Context is important here.
“84% of apps are free, and developers pay nothing to Apple.
Like any fair marketplace, developers decide what they want to charge from a set of price tiers. We only collect a commission from developers when a digital good or service is delivered through an app. Here are some of the ways developers commonly make money on the App Store.”
They’re talking about apps, and the second part of that header is referring the subject of that which is apps. They pay nothing because it’s a free app which generates no revenue to Apple (commission-wise).