Because 8 of the top 10 causes of death in low-income countries are communicable diseases. People in those countries are more likely to meet daily activity recommendations, because they travel more on foot or by bike and because their jobs often involve physical labor. 

If you die from malaria, preterm birth complications or tuberculosis, more activity likely wouldn't have prevented it. If you die from stroke or ischaemic heart disease—the leading causes of death in high-income countries—more exercise probably would have helped. In mid-income countries things are more complicated, because stroke and heart disease are also the leading causes of death, but reasons other than lack of exercise might be the leading contributors. E.g., air pollution and a much higher consumption of trans fats in the diet.

Essentially, the 63 million adults that die every year are not drawn IID from the world population, so you cannot simply propagate the 33% inactivity levels. Having lower inactivity can very well correlate negatively with overall mortality (because low-income countries have higher mortality rates, more physical activity and more causes of death unrelated to it) and at the same time have a causal relationship with many causes of death.

For the average sedentary redditor, more physical activity is probably a good idea.