That's because the "humans can read" part is not visible to all humans. Like myself I thought the image I could see was the one the machine couldn't. I managed to extract both forms of text by extremely crude means.
What this is, is a message that is more reliably readable by a sufficiently good AI than it is by all humans.
What this is, is a message that is more reliably readable by a sufficiently good AI than it is by all humans.