Same category error as always. "Screen time" is meaningless noise. They could be spending eight hours a day in the Kindle app reading classics. A good-faith study would evaluate what the kids are doing, not the tools they're using to do it.
Have you been around kids recently? I think we all know what they're doing on the phone, and it's not The Great Gatsby.
The numbers here are so overdetermined that it will never be possible to do a causal analysis, and jumping to "screen time" is almost outlandishly unjustified.