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)