What you are saying about signal server is true as long as you trust that they are doing things the way they tell you. (E.g. run a server that is similar to the sources they publish).
N.B.: I am not defending Telegram here, only saying that signal has become redundant now that they cannot claim server transparency anymore. On top of that, there is the unbearable attitude of its developers, who fight against decentralization (as a general concept), possibly on behalf of someone else leveraging on their charisma on a certain community, and strongly opposing to alternatives to G push notifications and playstore distribution. Even the telegram-gpl client is better than that.