The link is to a discussion: >Sigh. That Stack Exchange answer is incredibly old and points to the flaws everyone knew in MTProto 1, which has been superseded

The link is to a discussion: >Sigh. That Stack Exchange answer is incredibly old and points to the flaws everyone knew in MTProto 1, which has been superseded by MTProto 2 for years. Telegram's 1 on 1 secret chats are quite simple and straightforward in design. It is unlikely that there are any security issues in there. Telegram has the same usability issue that the rest of these end to end encrypted instant messengers have. They base all the security on having the users compare some ridiculously long numbers and then downplay the whole thing.