I think here we can assume it's not someone completely ignorant in the field(they did mention "outsiders in analogous fields"), so it's really about someone with a good understanding of an adjacent field, whose skills and knowledge can often be applied to the field in question, like bringing a computer scientist to robotics or biology, not like asking an accountant to design a rocket.
All fields share some amount of border. The amount varies, but there's no such binary thing as adjacent or not, and the important insight you need may come from absolutely anywhere.
