Do you have any source for this? As someone born in the summer to a farming family with poor eyesight, I find it hard to believe that happened because I wasn't exposed to enough sun as an infant or child.
I've worn glasses since I was 2.
I just so happen to have Hyperopia with astigmatism, neither of which came from a lack of outdoor exposure. (If anything, I needed less time outside).
That's a bit of the issue I have with such a broad generalization. It's true that for some, a lack of time outdoors damaged their eyesight, it's not universally true that all or perhaps even most poor eyesight is a result of staying indoors.