I don't get it...

From here https://telegram.org/faq#q-how-are-secret-chats-different

""" Q: Why not just make all chats ‘secret’? All Telegram messages are always securely encrypted. Messages in Secret Chats use client-client encryption, while Cloud Chats use client-server/server-client encryption and are stored encrypted in the Telegram Cloud (more here). This enables your cloud messages to be both secure and immediately accessible from any of your devices – even if you lose your device altogether. """

They claim that it's encrypted, but that uses a different kind of encryption.

Maybe this has changed recently?