But in theory, someone controlling a walled garden could do the same that youtube does with any kind of content. If the app doesn't work in a transparent manner anymore, you can just shoehorn unblockable ads into it.

Sort of, it’d be tough though. YouTube ads aren’t blocked because they’re not using a third party ad vendor. They’re big enough that they run their own auctions. The vast majority of services aren’t capable of creating the auction themselves, so their ads can be blocked.