Well they can do all that with a 10% fee too. The author is contending that by adding those restrictions and a high 30% fee, Apple is acting like a monopoly - it's their way or the highway.
When the App Store was launched, Steve Jobs said they plan to run it at break-even, no profit, and for quite a few years that was exactly what they did. Now they are profiting from the App Store and clearly could continue to run it as they do on a lower percentage, but I don't think 10% is that number. If they could run it on 10%, they wouldn't have operated it without profit for the first quite-a-few years at 30%!
Because of the lack of numbers, I'm not sure how long it was before running at break-even ceased to be a thing, though.
30% of subscriptions on the other hand might not.
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).
> For apps and in-app products offered through Google Play, the service fee is equivalent to 30% of the price. You receive 70% of the payment. The remaining 30% goes to the distribution partner and operating fees. [https://support.google.com/googleplay/android-developer/answ...]
But it's easy to argue that many of these just derived their rates from what existing similar store fees were. Epic Game Store tried to disrupt this and they're still at it a few years later but for most developers publishing on Steam/GOG nothing changed.