The causation could easily run the other way. In particular kids that have worse mental health use phones as a coping mechanism. What kind of kid can resist the lure of a phone until she 18? Only one with very strong mental fortitude.
The question being used to derive this data is: > “At what age did you get your own smartphone or tablet (e.g. iPad) with Internet access that you could carry with you?” 6 year olds can't acquire smartphones on their own. So there is definitely an element of parental agency here.
So the study is biased: they cannot conclude it's due to the smartphone and not the family environment, as both are different in the two samples.