But this also kind of feels to me like something that could go the other way- myopic kids are less likely to go outside

It’s not just individual kids they are measuring, but entire populations of children and at what age they start school. The younger kids are when they start school, the more likely they are to be myopic, and this contributes to significant differences in the prevalence in myopia across countries (Edit: I should have stated this explicitly, but this is because more time in school means less time outside, generally speaking)