Note it's not about impostor syndrome, I guess lots of us think we're in this category, that doesn't make it real.
Anyway, from the management point of view this is indeed a baffling situation when you encounter someone like this. What to do? Definitely not always easy, sometimes induced by the organization as well so it would be too easy to blame on the person alone. We need to pay more attention to those and there's clearly no magical recipe to handle them.