In my experience these people do not suffer from impostor syndrome. Although I have to admit that it could be a coping mechanism, it sounds more like Dunning-Kruger. I guess there are different symptoms of impostor syndrome. For me the comment of OP definitely resonated a lot. Not being able to express the things you know and don't know clearly blurs the lines for me to accurately value the knowledge I actually do possess or have mastered. :-)
I found an English translation here:
- https://w4nderlu.st/media/pages/publications/combinatorics-o...
Translated by Piero Molino at https://w4nderlu.st/publications