"Firstly, cannabis use is associated with an increased risk of experiencing schizophrenia symptoms, even after psychotic symptoms preceding the onset of cannabis use are controlled for, indicating that cannabis use is not secondary to a pre-existing psychosis."

I think that is saying that cannibis use was found to increase the risk of schizophrenia, whether or not the individual had shown indications of psychosis prior to the use of cannibis. As I get it, the site acknowledges that prior studies had left open the question of whether those subjects who developed schizophrenia had mental disorders prior to using cannibis.