Earlier quoted context omitted.
Telegram's secret chats are E2E encrypted.
Using an unexamined and unpublished encryption algorithm. Encryption is hard and when someone says "trust us", you know they are implementing poor encryption.
As someone professionally involved in cyber security, I fully understand and agree with the criticism that the protocol is non-standard and does not follow several best practices. On the other hand, it cannot be ignored that it hasn't been cracked yet, despite Telegram being one of the bigger messaging services in the world (especially one attracting a tech-savvy audience) and receiving a lot of attention.
The very least Telegram-haters could do is acknowledge Whatsapp's equally big problem: we cannot verify a thing. Facebok could have either open sourced the clients or published the outer shell of the wire protocol so we can verify the E2E encryption. They chose to do neither.