There is already a precedent for apple to not allow apps that doesn't align with its content policies even if user generated. Anything NSFW is not allowed by default and without explicit warnings. Porn is not allowed. Drug stores are not allowed. Gab is not allowed. Imageboards aren't. Why would this be different?

Apple owns the app store and the iOS platform. They have a duty to protect their customers from clicking on unsightly and wrongful content. Customers expect that from apple. I wouldn't want any kid to end up accessing this channel on telegram.