Once sold, an Apple device is no longer the property of Apple. Maintaining exclusive access to the primary (and I'd argue only) means of purchasing software for a device they do not own is a monopoly, even if it might not have been classically considered as such.
Perhaps they bought the device based on the primary metric of security and privacy. But with the banning of Telegram, it turns out the potential for secure chat is insufficient on iOS. Therefore, Apple is falsely advertising themselves as a privacy-focused platform. [0]
Therefore, the legal options available to Apple are either 1) open up their platform, 2) permit Telegram to be sold on the App Store, or 3) rescind their false advertising. The ball is in the EU court.
> Maintaining exclusive access to the primary (and I'd argue only) means of purchasing software for a device they do not own is a monopoly
The device is yours, the operating system is not, it has its own license and you even get regular updates of its terms. If you don't agree with them, you are free to jailbreak your device and use it at your own discretion & expense.
I hope the investigation ends in telegrams favor.