The graphs they show don't perfectly correlate to the trends in hanging out in person. While smartphones have definitely contributed to changes in the culture (for everyone, not just teens), it's false to pretend that they are entirely responsible for changes in loneliness. It just as well may correlate to being bombarded by reactionary hatred from adults over things teens have grown up thinking about more openly, like more open ideas about gender and sexuality. Or perhaps from finding themselves in a culture of fear about mass shootings while again faced with an adult political establishment who does nothing about it. Or maybe just facing the reality that the grown-ups are making things worse, day by day, and that they're going to have to clean this all up after us.
Smartphones have nothing to do with that.