Get better informed, secret chats work just like whatsapp, it has nothing to do with self-destructing messages, channels, groups and normal chats are its Telegram server and in fact you can also edit messages, while if you use secret chat you have e2e encryption and you can't edit messages and chat is on your mobile and it's not shared on all devices like everything else, I know because some secret chats that I have on one device are not on others. This is the text taken from the Telegram manual.
"there is end-to-end encryption on Telegram, just enable it: the chats on which encryption is enabled are called secret chats. In secret chats, text messages, images, videos and all other files are sent by end-to-end encryption."
Let's not disinformation please....