I agree my summary was a bit breezy. But I think an essential component of their theory is the focus on domain knowledge and metacognition, not skill as it is normally used. Consider the title of their first paper: "Unskilled and Unaware of It: How Difficulties in Recognizing One's Own Incompetence Lead to Inflated Self-Assessments". In the conclusion, they write, "We propose that those with limited knowledge in a domain suffer a dual burden: Not only do they reach mistaken conclusions and make regrettable errors, but their incompetence robs them of the ability to realize it."
lower skill -> higher confidence in ability (overestimation) higher skill -> lower confidence in ability (underestimation)