That is only true for the End 2 End encrypted chats, which are a separate and not very user friendly thing. Regular chats and group chats/channels are by design unencrypted.
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?
They have complete access to the data.