This statement is (arguably) true because so many of the things that cause Americans to be in poor health come as a result of behaviors, not as a result of illness or disease. While it is true that science might be able to handle the symptoms of some of these behaviors, Americans would be healthier if they would behave in healthier ways. Social policy can help to move Americans towards such healthier behaviors.
Many of the health problems Americans face come from obesity. They come from bad eating habits and lack of exercise, both of which lead to obesity. It is possible (though not easy) for social policy to encourage more exercise and better eating habits. This is more likely to help with things like diabetes, heart disease, and high blood pressure than science is.
For example, the government could push to have more restaurants put nutritional information on their menus. The government cannot force people to eat well, but it could give them more of the information that they need to make better food choices. The government (in this case, we are talking mostly about local governments) could do things to make it more convenient for people to get exercise. It could do things with city planning to create more walkable communities. The government could also do more to promote exercise through such things as expanded physical education programs in schools.
All of these things are social policies that might do more to address the root causes of American health problems, thus doing more than science can to improve the health of the average American.