I don’t think you need a particular depth of knowledge. You need someone willing to do the pre-work -clearly there are notes from and conversations with the contributors beforehand, someone with curiosity to learn about things outside their areas of expertise. A good journalist would be fine
He very often brings other knowledge to bear in conversations that clearly wasn't just prep work for the specific topic. He's not strong on science but he has a firm grounding in literature, philosophy and history that isn't common these days.