It's not just the issue of actual "alternative app stores". The issue that Telegram is running into here is that what Apple considers an "app store" is very broad, and when Telegram added games, Apple said "nope, can't do that, that would be an app store and that's illegal".

Facebook is running into the same issue trying to launch their Facebook Gaming app on iOS. Apple keeps rejecting them because it's an app store, according to their policies.

https://www.nytimes.com/2020/06/18/technology/apple-ios-face...