I only glanced at the paper, but I wonder how much of this is explained by just random chance?

It looks like they used multiple choice quizzes to determine both knowledge in science and a propensity to respond "don't know" indicating confidence. Any "don't know" response was counted as an incorrect response, while a correct guess increased the participants "science knowledge".

Thus, a willingness to guess something at random in the multiple choice test would both increase "science knowledge" as well as make the participant appear overconfident.