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

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.