Suppose you make the argument that developers, not consumers, pay for the dev kits, language, development, improvement, apis of the os. 30% might appropriately reflect the amount of work in an app that is apples work.
Services net sales in 2019 was $46.2B, and the cost of those sales was $16.8B (source 2019 10-K). They dont specifically break out App store vs other services, but as a whole the services section of their business is very, very profitable, according to their accounting. It remains so even if you throw in the entirety of R&D ($16.2B).
30% of subscriptions on the other hand might not.