>If it was indeed a backdoor, sure, but that's a judgment call, not something anyone knows. As others have noted, e2e was a novelty at the time, not a norm, and the platform itself was extremely new (less than a year old), and their stated reason for this was to protect against weak client RNG, which in retrospect sounds like a weak reason, but looking back at the news of 2013, this was right around the time the Snowden leaks caused everyone to believe RDRAND could indeed be compromised, so "client having state-compromised RNG" was indeed something on everyone's mind.

Everything you said here was addressed by the OP. The connection to telegram servers is already encrypted, the only adversary this server-side RNG could possibly defend against is one that has access to the server.