I am no cryptography expert. I judge by all the times I've seen programmers imagine they could do professional cryptography by themselves. Literally every time they fail. Thus, in my eyes it is more likely that Telegram's coders fell victim to the same illusion.

But I am not denying that it's possible it's the [beginnings of a] backdoor. The whole sub-thread is (a) my opinion on what's more likely and (b) calling out people who act snarky, offer no facts and demonstrate general negative bias.