Please read the link I provided before[0], it features links to a bunch of people saying Telegram is not secure[1][2][3][4]. It's not just Moxie. I actually have not seen any expert saying that Telegram is secure, but I'm interested in such thing. As the Telegram protocol is open, everybody is able to see how flawed it is. We know that. With a closed client that allegedly implements a secure encryption method (we also know it's good because it's open as well) there's a chance it doesn't work the way it's supposed to work, but there's also the chance it's working as intended. I'd rather take one percent chance over zero.
[0] https://security.stackexchange.com/questions/49782/is-telegr...
[1] https://eprint.iacr.org/2015/1177.pdf
[2] https://unhandledexpression.com/2013/12/17/telegram-stand-ba...
[3] http://www.alexrad.me/discourse/a-264-attack-on-telegram-and...