There is a world wide epidemic of myopia especially in Asia where there are more myopes than non-myopes. All factors pale in the cause of myopia beside the hours and hours that people spend focusing on books, computers, iPhones, and video screens. In the study you cite it is very preliminary and no conclusions can be drawn. The best evidence suggests for children and the young spending several hours/day outdoors is the best prevention. In China they are even constructing classrooms with glass walls so the child can look "far off" and cut down time spend on non-essential video screens.