They're simulating randomness incorrectly: relationship between true and perceived will average 0.5, not 0; and bias will average 50%, not 0%. That's why their "random data" is sloped.

Add negative relationship and negative bias, and the random data will act as intended - hovering randomly around 50%.

They, themselves, show the Dunning-Kruger effect by overestimating how much they understand randomness