Except he (or his corporation) has keys for almost all initiated chats on the Telegram network. Only the private chats are E2EE and they're not default and rather inconvenient because they don't sync between devices (unlike Signal's E2EE chats).
> To protect the data that is not covered by end-to-end encryption, Telegram uses a distributed infrastructure. Cloud chat data is stored in multiple data centers around the globe that are controlled by different legal entities spread across different jurisdictions. The relevant decryption keys are split into parts and are never kept in the same place as the data they protect. As a result, several court orders from different jurisdictions are required to force us to give up any data.
> Thanks to this structure, we can ensure that no single government or block of like-minded countries can intrude on people's privacy and freedom of expression. Telegram can be forced to give up data only if an issue is grave and universal enough to pass the scrutiny of several different legal systems around the world.
But if it was so weak then various intelligence agencies wouldn't be bothering with it
So we actually don't know if it's weak or not.
There is a difference between can't and won't.
They just make it hard to force them but they could if they want to.
Proper encrypted message can't be decrypted no matter how much they want to.
When the limitation is not technical, your only guarantee is the integrity of the operators and the lack of interest of your attackers coercing the operators into spying on you.
In Telegram's case one could easily assume that the Russians and now the French said him "Nice fortune you built, you are very successful person with a long life to live ahead. It would be a shame if you lose your fortune or spent your days locked in a room instead of flying on a private jet to Ibiza"