Ok, then I'll speak specifically about my local institution. My local art museum gets zero funding from taxes, but runs on its endowment and whatever revenue it brings in from admission, membership, events, food and beverage sales, and merchandise sales. Are they unsustainable because of that?
You're saying that a company can't be sustainable if its spending exceeds revenue, but there are plenty of institutions that do exactly that and are based on the same premise as what we know about Telegram's funding. A single (or multiple) large donor(s) can potentially donate enough money to make a thing sustainable without relying on much if any revenue. There are examples of that in the real world.
