> But the index of those feeds (the podcast search engine essentially - how podcast apps turn 'Three Dudes Talking' into a libsyn feed url - is hosted by Apple.

I do not know what you are talking about here. Apple has a podcast index. Podcasts do not need Apple's podcast index for anything. You can find a feed by going to the podcast's site and clicking on it, or googling the name of the podcast + rss. Or in any number of ways, really. Apple is one of many podcast indexes.

So no part of a podcast is hosted by Apple, unless the podcast is actually an Apple podcast.

Most podcast apps rely on Apple's podcast index to power search. Yes, you can manually add a feed in most clients but that is not how most people use their podcast app. They search for podcasts in their app of choice. If a podcast doesn't appear there because it isn't included in Apple's directory very few people will know that they can manually add a feed URL. Or understand any of those words.