What a gazillion pop-sci articles and blog posts suggest is that there is actually a negative correlation between self-assessment and performance -- i.e., that the higher people rate themselves, the worse they will perform. Looking at the graph we can see that while poor performers overestimate their performance relative to reality, there is still a positive correlation between performance and self-assessment.
Each figure shows how the bottom quartile is overestimating their ability. And that's exactly what people call the Dunning-Kruger effect.