That's a popular lie, Telegram uses the MTProto 2.0 Cloud algorithm for non-secret chats, which is audited and verified by multiple independent parties. For example WhatsApp claims it uses EE2E encrypted chats, how ever these claims are unverified and not audited. Also their chief executives are not in jail, coincidentally.
You can consult these links if you want to read more about it:
https://core.telegram.org/mtproto/AJiEAwIYFoAsBGJBjZwYoQIwFM...