Naively I would expect studies like this to be particularly sensitive to cognitive changes in young adulthood, but the authors did not adequately discuss the limitations of using undergraduates - I understand there are supply-side reasons for this, but it should have been addressed in the discussion. These students have not experienced much life outside of school, and their prefrontal cortices aren't even mature yet, so there are environmental and biological reasons to suppose undergraduates aren't representative of the general population. (This is true for any study but I think it's especially salient for metacognition and psychological confidence. Youths are notoriously overconfident.)
It would be interesting to see this experiment repeated with different age groups, and fascinating to see a long-term study that keeps track of individuals. This study is a bit old, I wonder if there were any follow-ups.