I don't know what is the reality, maybe indeed the smartphone is causing the problem. But the author is just jumping to the conclusion without proper scientifically caution.

There are a lot of other hypothesis that explain exactly the same observations, and that are not far-fetched (as you say, there is none _according to the author_, who then list all of these hypotheses in his rebutal). In his article, he goes through a lot of them but don't address them properly. For example, the more recent social anxiety about climate change is just "I don't think so".

Personally, if I have to bet, I think it is social media. But then, 1) I will not claim "it is social media", just "it may be social media", 2) it would be stupid of me to then isolate smartphone: if it is social media, then forbidding smartphone but not the laptops would be totally useless (and if the argument is turned into "no but the problem is the constant usage all the time, which you cannot do with a laptop", then, again, it's just pure conjecture: it may end up being true, but it is still scientifically incorrect to present these conclusions as scientific)