They do not. This entire case is about them not being able to use in app purchases (in which case they would pay 30%) or steer people to their site to pay for subscriptions. Spotify doesn’t sell subscriptions from the apps, and thus they pay nothing.
I thought that you couldn't subscribe to Spotify via the App. Am I wrong?
You are correct, and Spotify can't steer people to a subscription flow outside of the app through the app to not infringe into App Store's policies either. That's one of the main issues, either you pay the 30% Apple tax or you can't even tell your users "click here to subscribe" to a checkout flow outside of the app and App Store.