These are all false claims. Messages are not stored in plaintext. Secret Chats are E2EE and use standard crypto primitives. All of this is well-documented. [1] [2]

[1] https://core.telegram.org/mtproto/AJiEAwIYFoAsBGJBjZwYoQIwFM...

[2] https://core.telegram.org/api/end-to-end