Dude, you are really being selective with your quoting. Immediately before that paragraph, they say your approach doesn't work

Although gun restriction laws that focus broadly on mental illness are an understandable initial reaction, they will be extremely low yield and wasteful of scant resources. Furthermore, such laws perpetuate the myth that mental illness leads to violence and gives the public the incorrect message that mental illness is significantly associated with gun violence directed toward others.

And immediately after:

Unfortunately, some disturbed individuals are likely to remain inaccessible to whatever interventions mental health professionals have to offer. This situation accounts, at least in part, for the fact that measures such as screening for prior psychiatric treatment (often in the distant past) among individuals who want to legally purchase firearms do not represent meaningful interventions (Brady Handgun Violence Prevention Act of 1993, Pub. L. No. 103-159, 18 U.S.C. § 922 [s1–s6]; Norris et al. 2006; Simpson 2007). Experience and research have demonstrated that more promising, higher-yield interventions include both 1) third-party reporting of warning behaviors or leaked intent and 2) social and media responsibility (Meloy and O’Toole 2011; Meloy et al. 2011; O’Toole 2014). On a fundamental level, the behavior and motives of mass shooters must be distinguished from psychiatric diagnoses. The belief that these categories overlap or have a direct causal association is not supported by available evidence. More importantly, interventions to decrease the morbidity and mortality of gun violence based on such overgeneralized views are not likely to be successful and may cause more harm than good.