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. :-)