"Telegram is not intended to bring revenue, it will never sell ads or accept outside investment. It also cannot be sold. We're not building a “user base”, we are building a messenger for the people."
https://telegram.org/faq#q-how-are-you-going-to-make-money-o...
I don't think you can trust an organization anymore than you can "trust" a weapon. A bad person could theoretically take control of it and use it for evil.
The closest you can get is trusting the individual at the helm. You know Automattic isn't going to do anything weird as long as Matt Mullenweg is the dictator.
Granted, there's nothing legally binding. But it's one of their selling points.