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.