It might be OK for one app to allow such content and it might be not OK for Telegram. Because Telegram earlier had issues with Apple [1] and if they violate Apple's rules again it would become a convenient excuse to remove Telegram from App Store.

You are trying to find a reasoning in Apple's decisions, but isn't it easier to assume that not all apps are equally welcome in a private app store?

[1] https://www.theverge.com/2018/5/31/17412396/telegram-apple-a...