Primarily yes, and while true - that's an extreme and a bit pointless, imo. I can say the same about Linux, which i'm on now, as i've never verified what i'm _actually_ running. Or Matrix servers i'm connected to, and what they're _actually_ running.
Yet the idea remains that something some people have analyzed is better than something "no one" (outside Telegram lol) has analyzed.
Your question feels as if we may as well all be running entirely closed source. I'm unsure why the benefits and/or supporting arguments for FOSS need to be stated here. Is there a degree of your question i'm missing?
edit: Oh and, of course, the FOSS nature is even more important in the case where the majority of Telegram communication seems to lack security. If this was a zero knowledge platform i'd be far less concerned about their serverside implementation.