Downloading addititional information could be made from secretive preexisted chanel,

Fair enough, that could work but there are limitations on this new channel in terms of its position (can't be close to the first signal) and the same limitations for message length. Additionally you need to send the key for the decryption of the second message with the first message.

They ask about it - but it is only a trick

Surely we will know where the message is originating from and will know how long it will take us to send them a message back and thus they cant trick us into believing that there is backwards communication when there isn't. Especially so when we see that the message stays the same and is repeating itself in every direction.