It isn't impostor syndrome, they are literally an imposter. They committed fraud to get in the door, you didn't coerce them to get a much better paying job tha

It isn't impostor syndrome, they are literally an imposter. They committed fraud to get in the door, you didn't coerce them to get a much better paying job than they are qualified for. Now that they know they can't do the job, they are content to keep cashing paychecks and hope that your company is too hand wringing and soft to call them out. To your face they are worried and "what will we do, oh no what a mistake we all made", behind your back they think you are suckers (and based on your post they are probably correct).