I’m glad he didn’t advise birdshot. While those loads COULD be effective, better to stick with #4 or bigger buck or slugs for social work. Also I’m not as big a fan of slugs in a smooth-bore shotgun simply because it takes a gun that’s good at being a shotgun and turning it into a gun that’s a lousy rifle.
Still there might be times when you need a lousy rifle, so I don’t see any reason why you shouldn’t keep a few slugs on a bandoleer or side-saddle or what have you.