(1) This study is specifically NOT making the claim "social media is definitely fine and good for kids," so really more like skepticism at anyone trying to draw a conclusion of "...and therefore we should let Instagram off the hook :)"

and

(2) There is a reasonable case to be made that a lot of adults say that social media makes them miserable, and therefore that probably extends to kids too, if not even more so.

Like, for example, stuff about teen suicides that are linked to various happenings on social media: it's true that those happen. It's also true that teen suicide rates are higher now than they were a few decades ago. It's not definitively true that they happen in higher numbers than they would've without social media (e.g., teen bullying just has a new outlet), but it's also fair to suspect that there might possibly maybe be a connection, even an indirect one.

No single study is definitive, including this one!