I am a bit surprised that there is no mention of academic performance, which would be the most obvious indication for a successful policy.

Instead, what the article basically says is that banning phones results in kids spending less time on the phone, which is an improvement if your idea of an improvement is having kids do things that don't involve spending time on the phone. There is some circular reasoning here.

Note that I am not against the idea that phone bans have positive effects on general well being, academic performance, etc... but for me, this article doesn't sell it very well.