You're ignoring opportunity cost which was the whole point here. You didn't say how much Apple was getting.
If Spotify charges $10 and pays Apple $3 then Spotify makes $7 and Apple makes $3.
If Apple enters the market and charges $10 then consider a user that would have otherwise went to Spotify. Spotify makes $0 and Apple makes $10 from the subscription but loses $3 from what they would have been paid from Spotify so Apple makes $7.
Apple has lots of advantages stemming from their control of the platform but "not having to pay the 30% fee" isn't one of them.
If Spotify charges $10 and pays Apple $3 then Spotify makes $7 and Apple makes $3.
If Apple enters the market and charges $10 then consider a user that would have otherwise went to Spotify. Spotify makes $0 and Apple makes $10 from the subscription but loses $3 from what they would have been paid from Spotify so Apple makes $7.
Apple has lots of advantages stemming from their control of the platform but "not having to pay the 30% fee" isn't one of them.