It's an interesting point - it may be possible to have 'impostor syndrome' syndrome, where you think you have imposter syndrome but you're actually an imposter. According to the study, if you were an imposter you'd have to be incompetent which means you'd be unable to judge that you were incompetent and you'd rationalize that you were competent.
Terrible cycle, but it's according to this article if you were an imposter you'd probably think you weren't. Uhm. This probably isn't helping. Good luck with your imposter syndrome!
For a long time, my biggest fear was that I was secretly retarded. Every time anyone said I was smart, I thought they were just being nice, like how you'd put even your child's ugliest drawing on the fridge.
Of course, if I were, I probably couldn't comprehend the thought in the first place, so rationally, I know I'm not. A lot of it probably comes from reading things from/about people infinitely smarter/more motivated than I am, and feeling like I have no idea what I'm doing.