How rewarding is an environment where everyone already likes the same things? I'd say a rewarding creative environment is one where different people bring different perspectives to the table to create new possibilities from the contrast and synergy. And an "in-group only" barrier to entry seems like the opposite of an open and welcoming work environment.
Fandom is a hobby. Writing is a profession. Having a hobby in common is not a meaningful qualification for professional work, and it's not a requirement for people to be able to work together comfortably. A pleasant work environment isn't about what people did before they got the job, it's about the climate the employers create within the job itself. If they need to have some prior connection to work together well, that means the people in charge are abdicating their own responsibility to nurture a positive climate and relationships. (And seriously, when have fans as a group ever gotten along without conflict?)