This is something I hadn't thought about until the article pointed it out. According to the first link in google (https://www.braze.com/blog/in-app-purchase-stats/) the average iOS IAP in 2017 was $1.08. If you compare that to Square and Stripe's fee structure then the processing fees charged by the credit card processors is about 25%. This means Apple's taking for themselves much closer to a 5% cut which seems much more reasonable for the services provided.
Why Apple doesn't switch to a 5% + $0.29 structure is anybody's guess, however...