Spotify seems to focus on IAP, but if you purchased Spotify Premium outside of IAP, it works just fine with that app.
I have to imagine a fair number of Google searches for "spotify remove ads", too, which can lead to an upgrade.
I guess the crutch of Spotify's argument is that 30% is too high to charge while having their own competing app that isn't required to pay the same fees. Effectively using their unilateral control of the App Store and IAP to squeeze a third party competitor
Spotify can do this thing called "advertising". Look into it.
They advertise for Spotify Premium constantly on their free tier.