More specifically, it's called the Dunning-Kruger effect. Impostor syndrome is a broader description that heavily overlaps with the "give up competing due to perceived losses" that the article talks about.
(As a side effect of this, one could predict that approaches to mitigation of impostor syndrome (which there are a few of) will be especially helpful if you're concerned about improving women's representation. Somewhat ironically though, the "ideological echo chamber" that is most active in pursuing these goals is not always very open to such notions!)
Saying a class is easy is not the Dunning-Kruger effect, especially when the class is easy for the individual. Thinking mastering the entire field is easy because the intro class was easy is a far better match of the Dunning-Kruger effect.
Someone who is well aware of how complex a subject is can find a given class to be easy, which would imply they are not under the Dunning-Kruger effect. Confidence alone is not sufficient.
Edit: that they ended up with lower marks due to not studying due to overconfidence due to the earlier ease of the topic seems like it would fall under Dunning-Kruger.
No, you have it backwards. Dunning-Kruger describes incompetent people thinking their work is high quality, or overestimating their level of competence. Impostor syndrome is when people feel inadequate despite producing acceptable work.
(As a side effect of this, one could predict that approaches to mitigation of impostor syndrome (which there are a few of) will be especially helpful if you're concerned about improving women's representation. Somewhat ironically though, the "ideological echo chamber" that is most active in pursuing these goals is not always very open to such notions!)