Nitpick: developers aren't required by Apple to restrict payment to only go through the App Store / Apple Pay. They're just required to take an equivalent App Store levy out of whatever gross they're making, regardless of what payment processor they use, and hand that levy to Apple.

For example, there's no TOS requirement preventing Amazon from having been selling Kindle books through its iOS Kindle app this whole time, and charging those sales through the user's Amazon payment processing as per usual. The only TOS requirement is that they would have just had to take 30% off the top of those sales for Apple. Amazon doesn't want to do this (and perhaps would have negative margins if they did), so instead they have decided to not have any kind of store view in the iOS Kindle app, and instead to just tell iOS Kindle users to go to the Amazon website to shop for Kindle books.