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.