They do not compare to the baseline population correctly. Roughly 1% of all people are susceptible to schizophrenia, and up to around 4% schizophrenia, extreme bipolar, and other conditions that can result in psychotic episodes or extreme outcomes.
Drug use among vulnerable populations increases the risk of psychotic episodes, but does not increase the risk of developing those conditions. There is no difference in the rate of extreme psychological outcomes among drug users and non-drug users, and in fact, this study reinforces that observation - only 4,000 of 460,000 had those negative outcomes. Over the next 20 years, it's extrmeely likely that another 600-1000 will develop schizophrenia, even abstaining from drugs entirely. Drug use can trigger a psychotic episode and result in long term schizophrenia; by the time you turn 45, however, your odds of a schizophrenic break drop to almost 0.
The worst part of drug use and mental health outcomes is that it can rob people of normal years of life, and rarely, result in schizophrenic or other psychotic conditions being triggered when they might never have been. However, this is not just marijuana or other illegal drugs, but alcohol, caffeine, trauma or intense stress, and even chronic health issues can have the same outcome.
This study also fails to account for the confounding fact that people with mental health issues often pursue mind altering drugs in order to self medicate. People with bad conditions in life, especially younger, undergo extreme stress and are exposed to illicit substances much more readily than those in otherwise stable and healthy conditions.
The results and methodology are flawed, and the conclusions being drawn have little to no relationship with reality.
It comes down to susceptibility - genetics and health conditions play into this. Consult a doctor, and if you have risk factors, live your life accordingly.
If you don't have risk factors for schizophrenia, drug use will not suddenly put you at risk of developing it. Marijuana or other recreational drug use will not cause you to have a psychotic episode. If you do have risk factors, then you're twice as likely to have an episode by using drugs or experiencing other triggers than otherwise.
For those who are susceptible, your relative risk of psychotic episodes and mental breakdown double under mairjuana and other substance use.
For those who are not susceptible, your absolute risk of psychotic episodes and mental breakdown remain near 0. Drugs don't induce these conditions (except in the case of extreme stimulant abuse, and possibly extreme psychedelics outcomes, although getting fried by psychedelics isn't really the same thing as psychosis. Lots of high function deadheads survived some truly harrowing levels of substance use and are best characterized as "weird".)
It'd be nice if the media could distinguish between relative and absolute risk rates and communicate the difference effectively. It'd be even nicer if researchers and publishers didn't chase clickbaity results like this and mischaracterize things like relative and absolute risk for profit.