Who knows what

Illustration of the idea behind Diffie-Hellman key Exchange

I'm reading about the Diffie–Hellman Key Exchange and I come across this phrase:

The chart below depicts who knows what,

Who knows what? Are you kidding me? Isn't that the phrase we use when we don't know what the subject is? Does this mean that whoever is writing the description of this chart does not know what it depicts? Well, that's dumb. How can you write a description of something you don't understand? But then I realize that the chart shows what information is known by which people, i.e. who knows what.