Many academics and students claim to be suffering from imposter syndrome. Many of these will be strong contributors regardless and I hope they find a way to cope with their self-esteem issues.
However, some of these actually will be imposters, and their feelings are appropiate and accurate indicators of their actual level of intellect and knowledge. Some of these people even claim to be suffering from imposter syndrome, when in fact their concerns are well justified.
What's a name for such people?
For example, there's a guy I know who often talks about how he has imposter syndrome because he often feels inadequate and outperformed in comparison to other students in our class. I don't know how to tell him that our class (in my experience) is wholly average and anybody who can't keep up with the level simply doesn't have the intellect/knowledge/effort required.