Transmission implies it is causing more mental disorders than would otherwise be present, regardless if they were identified or not.

I wouldn't assume that, particularly when the study is very careful to always say it led to an increase in diagnosis.

And really, 'I dislike the term they used to describe how diagnosis spread' is fairly nitpicky.

The title of this study is "Transmission of Mental Disorders in Adolescent Peer Networks". The "Meaning" section states "The findings of this study suggest that mental disorders might be socially transmitted within adolescent peer networks." I can't believe any plain read of that would imply they're talking about increased diagnosis of already-present mental illness due to greater awareness.