Yes, the Apple-Google duopoly poses a much bigger problem for freedoms than Twitter (I covered that in detail last summer https://teleg.eu/s/durov/122). Apple is the more dangerous of the two because it can completely restrict which apps you use, while on Android you can install self-hosted apps as APKs. That's why I've been urging users to switch from iOS to Android - that's the least they can do to retain access to a free flow of information.
As for iOS, we are working on a feature-rich web app which will run in Safari almost as smoothly as the native app. Still not the same thing, but would work in extreme cases if for any reasons .
Having said that, we don't see why Apple or Google can be unhappy with Teleg.eu, so this discussion is purely hypothetical.