I take him seriously. The slow carb diet from 4 Hour Body has had a huge impact on how I eat. It works and has helped 1000's of people improve their lives.
Quirk diets work because adherents accidentally get the calorie balance in the right direction by limiting/changing dietary habits. It's not the quirk, which might maybe contribute a tiny effect if it's not entirely bogus in principle. Yet people hail, "The diet works! The guru was right! All hail the Guru!"
Ask and you'll hear the exact same refrain for homeopathy, acupuncture, rhino horn powder, shark fin soup, albino body parts, ...