And they aren't even allowed to only charge Apple Users 30% more
What's not allowed is telling the users that it's available for cheaper in other places. But you can charge as much as you want.
Spotify is free to link outside the app and inform of better deals.
Spotify may apply for permission to have one link that does "not include, or be used with, language that includes the price of items available on the website". The OS will throw up an intermediary scare sheet warning the users of the dangers of the link https://developer.apple.com/support/reader-apps/