However, study limitations prevent proving causality, though reduced outdoor exposure is likely a key contributing factor.
This. The hypothesis before this study was that disproportionate time spent focusing exclusively on close things (reading, drawing, crafting), without corresponding shifts to distant objects, led to nearsightedness. Screen time is just another close-focus activity, one that has skyrocketed in practice since the days of "don't sit too close to the TV, you'll [insert bogus malady here]."