There’s a difference between screen time and social media usage.

FTA

"One also suggested we take a look at social media on its own because it’s a source of worry for many and we did not find anything special about this form of online engagement.”

It's a subset of what they studied. Surely a lot of screen time is spent on social media. They did not observe anything special about time spent on social media. A more in-depth study may yield more insight but there is no thread to pull on from this study. Only parents' misgivings.