That's not true at all. They're not claiming they can't split it up technically. They're claiming that part of the end-to-end user experience is that users trus

That's not true at all. They're not claiming they can't split it up technically. They're claiming that part of the end-to-end user experience is that users trust the App Store because Apple has oversight into the entire chain from downloading apps to the OS to taking payment information on their devices. If they have to open it up to other parties, then that chain of trust is broken.