Dubious at best. If one has children they notice quite visibly that screen time makes their children more agitated during the days they have screen time. And screen time is as addictive to children as it is to adults, and we should consider the effects they have on their developing brains.
When science doesn't match your personal observations, that doesn't make the science wrong. It also doesn't make your personal observations wrong - it usually (in my experience) means that you're measuring/observing different things (assuming you're reasonably intelligent, which I am doing). In this case, "cognitive development and well being" are complex concepts that probably do not map exactly (or maybe even gross…
And in this case the quantifiable, non-self-reported part is the MRI brain scans they're doing, but what does that data indicate? I'm too ignorant of neuroscience to know. Are all changes in behavior and cognition detectable through an MRI scan? Maybe the average kid who spends 8 hours a day playing video games finds it nearly impossible to read a single page of a Harry Potter book without getting distracted, while a kid with 2 average hours of screen time has no problem with it, yet the difference maybe isn't reflected in an MRI scan? Did they perform these same scans on some Amish kids as a control?
Point being sometimes studies like this are hard to swallow when their conclusions go against so much anecdotal observation, particularly when the methodology leaves room for all kinds of other interpretations