(I grew up on a farm, and spent a helluva lot of time outside, and needed glasses long before my family had a computer; my father would have spent even more time outside than I did, and likewise is pretty nearsighted.)
1. Myopia is not entirely genetic, because genes for myopia could not have spread to almost all Koreans from almost no Koreans in the span of 56 years.
2. Myopia is somewhat genetic, because myopia existed in a significant proportion of the pre-industrial population.
I am curious whether 18% was the diagnosed rate of myopia in South Korea in 1955 or the actual rate of myopia in 1955.
In 1955 South Korea was a poor country; now it is one of the wealthier countries (per capita) in the world. On the one hand, it is entirely believable to me that access to eye correction is radically greater than ~70 years ago, and there was a lot of undiagnosed vision problems in 1955. On the other hand, it's also entirely believable to me that literally every 20 year old (male) was given a vision test in 1955 in South Korea, because of the War. On the third hand, it's also entirely believable to me that the vision test given was fairly easy to "pass", and that a many of the people who passed it would have a level of myopia that we would now prescribe glasses for, because hey, better vision is better.
That conclusion is can't be supported from the above fact alone.
It could be a development disorder with a certain probability of occurrence given other non-genetic factors.
Perhaps some pre-industrial people didn't get enough sun time, because they spent too time at home reading. Others had physical eye damage from work activities, etc.
I am not a doctor, but imo it is similar to a lot of other health concerns. Predominantly genetic, but you can still affect the outcome in slight ways and improve your chances a bit.
Personal example: my grandfather who has been smoking at least a pack a day since the age of 9, and he is still doing way better in his 80s than majority of people his age. Working in the garden (even physically demanding stuff, like preparing the soil for potatoes every summer), fully mentally sound, etc. On another hand, you have plenty of young people who do everything right (no smoking, healthy diet, regular proper exercise, etc.) falling due to random health ailments. Which goes to show that while taking care of yourself is important and beneficial, the luck of hand you got can override it all in either direction at any moment.
Back when I first moved into my current place in 2013, I was able to read my stove's clock from across two rooms. My eyesight had steadily degraded since then until last year/early this year, where I needed to step into the closer room before I was able to read it.
...But then, the lockdown started. And since then, despite being on the computer for more of the day and not often going out into the sunlight, my eyesight has been improving. I can clearly read the clock from across both rooms again.
Since I don't really open my blinds, I've gotten pretty much continuous indoor levels of light for two months now, so I've my own theory: It's not specifically sunlight, but having to adjust to the differing brightness between indoor and outdoor light too often that causes issues.
I was outside. If what a small child running around on a farm for hours and hours every day, rain or shine, doesn't count as "being outside more", we need to start using a different word to refer to that.