Difficult for an ordinary person to evaluate this kind of study. The most I can do is give my own subjective opinions. And I tend to disagree. If anything smartphones may in fact make us smarter, or at least better informed.
I was using a Palm PDA back in the 1990s for organizer duties like addresses, phone numbers, calendar and to-do items. It was my "extrasomatic memory" (supplemental memory outside my own brain), my digital little black book. Then I transitioned those functions to a smartphone.
And I'll tell yah, using these devices, including my computer and iPad, takes some learning. I sometimes wonder if I have to put as much brain power into remembering how to utilize all their features and software as the benefits I get back out of them. Jotting entries into a little black book was never this complicated!
Only now my black book is of almost infinite size, and info can be instantly located and retrieved. IF I know how to ask the right question, sometimes an art in itself, and how to be skeptical of some of the answers, using multiple sources to crosscheck some material, which this article mentions.
But I don't feel I'm suffering from lazy thinking, just utilizing new skill sets, in conjunction with a digital and online environment. And I feel more empowered & productive.