Sometimes after the research is done, we get an non-intuitive result.
Now we can use these results to make policy and rules better for the students, or we can just double down on our own view and stick our heads in the sand, and just push what we think will help despite the data.
That said while as an old guy, who went to school before everyone had cell phones. Heck in my under grad, I was one of the few students with a cell phone that just made calls (it did text too, but I had no one else to text to, plus it was expensive)
When I was in grad school I was old and mature enough not to use my phone during class.
So I would think having a smartphone in school with kids who are already easily distracted would cause more problems. But with the data results, I think I answered why I was was, kids during school age are easily distracted by anything. Doodling in your note book, checking out the pretty girl in front of you, joking around with your friends without trying to get caught... I think it is the case during school and being school age you are extremely distracted by everything anyways, having a phone in school wouldn't be much different, you would just be replacing say one of your activities to distract yourself with the phone. Perhaps less doodles in the notebook, (as I wouldn't imagine, that my phone would distract me from a pretty girl who I had a crush on).
But the results about social media, which is a big after school activity I feel would be the bigger factor. As I stated I am now an Old Guy, and I have to make sure I don't get too involved in social media myself, as you can go down a Toxic hole of misinformation and annoyingly stupid people and comments, that repeat over and over again... For a kid being exposed to that earlier in life, it would like being stuck in school drama even out of school, where you can normally relax from it and actually feel like a human for a bit.