which has been proven to be less than 100% airtight and secure.

I don’t believe that has been proven. There has been criticism of it 1, 2 from prominent cryptographers though.

Telegram’s MTProto protocol isn’t obviously broken in a practical way, concedes Matt Green, a cryptographer at Johns Hopkins University who has consulted for Facebook on encrypted messaging systems. But it’s uniquely “weird,” he says, in a way that suggests its inventors don’t understand tried-and-true cryptography practices and raises his suspicions that it may yet have undiscovered vulnerabilities.

Their response was even more dodgy trying to somehow inject some sort of “nationalistic”, “america bad” into it:

Telegram’s Ravdonikas argues that “Telegram encryption relies on classical algorithms, because we consider some approaches promoted by US-based cryptographers after 9-11/the Patriot Act (which your sources refer to as ‘state of the art cryptography’) questionable."

At the end of the day math is math regardless where it comes from. Secret chats also only work with the mobile client, have to be manually turned on and do not work for group chats and as it’s a centralized server you can’t host your own.