MTProto is not end-to-end. MTProto is their obfuscated client-server transport encryption.
What the commenter above is referring to is Telegram defaulting to saving your messages on the server in plaintext. You can use a “secret chat” which enables end-to-end encryption, but that is separate from MTProto.
Your sentiment is correct though. Messages should not be visible in plaintext to the server.
I dont know much about it, but Wikipedia says that MTProto is specifically for “secret chats”:
For encrypted chats (branded as Secret Chats), Telegram uses a custom-built symmetric encryption scheme called MTProto.
https://en.m.wikipedia.org/wiki/Telegram_(software)#Architecture
Maybe Wikipedia is misleading here
You’re right, it is misleading. There are different “flavours” of MTProto. See here:
https://core.telegram.org/mtproto
This page deals with the basic layer of MTProto encryption used for Cloud chats (server-client encryption). See also:
Secret chats, end-to-end-encryption
End-to-end encrypted Voice Calls
(The major difference is simply whether the server and client share a key or two clients)
