Then it is not correct. A security analysis of Telegram has raised doubts about their e2e before. They have (had?) a blogpost up where they pontificate about how it not necessary and would put a burden on their channel feature anyway.
The security analyses I am aware of target MTProto 1 which is not MTProto 2 used for quite a number of years now. MTProto 2 uses standard security primitives.
You cannot say "it is not correct" without proof. We know all messages not just E2EE are wrapped in encryption on their way out a Telegram client and we know they are decrypted on their way in because client source code is available.
Why don't you have a look for yourself then let us know if you've confirmed your suppositions?