That's the thing that jumped out to me here—iPhones themselves and everything you could actually do with one during that period were way different from today. There were effectively no endless feeds of content to consume, the phone screens were way smaller and less vibrant, push notifications only came out about halfway through.

I was age 19-23 during that period (in the "highest impact" age group from the article), and I think I used my phone more for coordinating in-person social activity than anything else at the time. Additionally on that—iPhones were not widespread in my cohort at the time, even at an expensive private college with many students from upper income families.

It really did start in the time frame. Facebook opened in 2006. Twitter already existed and grew rapidly in that period. There were messengers. And because everything was slow, I suppose it could even take an hour to doom scroll Slashdot.

Mobile gaming might be another reason.