I don't have any real evidence, but anecdotally I suspect that diet also has an effect on vision. Could be just a coincidence, but my vision declined as a child during a period when I was regularly stuffing myself with sweets, and know others with similar experience.
Is the diet in East Asian countries (Hong Kong, Taiwan, Singapore & South Korea) significantly worse than it was in the 1920s? As the article details, myopia is increasingly common in China as well. The Chinese suffered numerous famines between 1920 and 1960.
I'm sure diet plays some role in eye health, but nutrition has likely improved in all of the countries they're discussing.
Another anecdote: I've consumed on average multiple cans of soda a day since I was in highschool, and while this habit has had a disastrous impact on my health I have always had exceptionally good vision (I can read the bottom line on the eye chart).
