While I would bet that the general idea that few people lead healthy lifestyles is correct, the study is somewhat misleading, given how they measured the "healthy" amount of physical activity [0]:

"While attending the MEC, participants were instructed to wear an ActiGraph 7164 accelerometer (ActiGraph LLC) during all activities except water-based activities and while sleeping. The accelerometer measured the frequency, intensity, and duration of physical activity by generating an activity count proportional to the measured acceleration."

and dietary behavior [1]:

"Data from the Dietary Data Collection system were used to compute a Healthy Eating Index (HEI) score for each study participant. The HEI-2005 [2] was developed by the US Department of Agriculture as an indicator of dietary quality. The HEI has 12 components (total fruit; whole fruit; total vegetable; dark green, orange vegetable and legumes; total grain; whole grain; milk; meat and beans; oil; saturated fats; sodium; and calories from solid fats, alcoholic beverages, and added sugars), with each component individually scored and a maximum total score of 100. A higher score reflects closer adherence to the dietary guidelines for Americans."

I would guess the number is not higher than the 5-10% range, but 2,7% seems too low. Let's say you swim 5x a week and follow a ketogenic/low-carb diet, the study would have you classified as unhealthy. The HEI-2005 [2] is already outdated (revised in 2010) and it is probably not the "optimal" diet either.

[0] http://www.mayoclinicproceedings.org/article/S0025-6196(16)0...

[1] http://www.mayoclinicproceedings.org/article/S0025-6196(16)0...

[2] http://epi.grants.cancer.gov/hei/2005/