He's arguing that the United States has had a stable political system for a long period of time because the federal government has relatively few powers, allowing the people in its widespread, culturally-different regions to craft state and local governments to their liking. I suspect he believes an activist, centralized government, imposed on all Americans, would eventually result in the division of the United States.
There's plenty of stuff in there to debate (the US did have a rather bloody civil war, so it's not like it's been perfectly stable), but his argument is plausible enough - I'm not sure why it makes no sense to you. Personally, traveling around the United States has convinced me that there's ten countries worth of distinct cultures in it - one-size-fits-all government isn't going to work well.
There's plenty of stuff in there to debate (the US did have a rather bloody civil war, so it's not like it's been perfectly stable), but his argument is plausible enough - I'm not sure why it makes no sense to you. Personally, traveling around the United States has convinced me that there's ten countries worth of distinct cultures in it - one-size-fits-all government isn't going to work well.