Harry actually says that can’t trust something simply because it mentions the recognition code in the chapter that mentions it:
Dear Me,
Please play the game. You can only play the game once in a lifetime. This is an irreplaceable opportunity.
Recognition code 927, I am a potato.
Yours,
You.
Harry nodded slowly. “Recognition code 927, I am a potato” was indeed the message he had worked out in advance—some years earlier, while watching TV—that only he would know. If he had to identify a duplicate of himself as being really him, or something. Just in case. Be Prepared.
Harry couldn’t trust the message, there might be other spells involved. But it ruled out any simple prank. He had definitely written this and he definitely didn’t remember writing it.
Harry actually says that can’t trust something simply because it mentions the recognition code in the chapter that mentions it: