Also, it makes no sense to assume that prior fandom is the only way to be knowledgeable about a subject. As I already said, in any other profession, if the job requires certain knowledge or skill, then you'll be given the necessary training before you start, or be required to get it yourself.
I agree with your overall point -- looking at his entire body of work, I'd argue Jack Kirby wasn't actually a superhero fan -- but plenty of professions make having a certain knowledge set or skill a -prerequisite- for hiring and wouldn't consider anyone who didn't already have it.