No, they didn't control for confounders. They did separate analysis based on grade level and whether English is the home language. But they didn't control for

No, they didn't control for confounders. They did separate analysis based on grade level and whether English is the home language. But they didn't control for parental education or socioeconomic status, both of which would likely be predictive of a child's reading comprehension and their chance of having a phone as a 9yo.