Apple is not the exclusive source of podcasts or ebooks on iOS or otherwise; they provide an optional store. Especially for podcasts their store is a new, optional feature and it is perfectly possible to offer your own store and your own authenticated podcasts and integrate them into Apples podcasts app. You can also offer them in Apples store and have them deal with payments and distribution and accept that they charge a 30% commission. They don’t even require exclusivity, you can both offer your own store and Apples store at the same time, whichever your customers find more convenient.