The survey seems to be sound in terms of methodology, sample size and tracking period. Like any statistical survey the findings might point to correlated variables that are main causes, but some variables are more difficult to ascertain what is behind them.
Lack or too much sleep might be result of hormonal or metabolic problems, also the result of breathing issues due to congenital defects or too much height.
So, although some variable aren't perfect they can me measured and tracked, while deeper and more granular issues are difficult to categorize and measure.
Cause the researchers are trying to triage main indicators of well-being and study their aftermath, so afterwards more research can be done on the causes of preventable death.
And for most part, the results are pretty much common sense. Smoking is bad, period. Heavy drinking kills your liver cells, depletes your vitamin B complex reserves, and dehydrates the body.
Chronic lack of sleep causes changes on the metabolism, increases stress response feedback, and increases the probability of an accident.
Sitting down for long periods might cause hemorrhoids and muscular atrophy, it also doesn't do much good to your bowels.
Everything put together, regardless of the network of causality that made it happen, will not have a good effect on your general health.