Genetic evidence suggest that the human population was bottlenecked to 3,000–10,000 individuals about 70,000 years ago. Not all that much time has passed since then, evolutionarily speaking, and whatever genetic predispositions were present in that tiny population are probably mostly still kicking around the gene pool. A disorder that doesn't cause problems before sexual maturity or that wouldn't pose an issue for a hunter-gatherer in the environment of evolutionary adaptation isn't really maladaptive, and there's no reason to suspect some number of such traits wouldn't be widespread.
It can also mean that the normal state of the brain exists near a small number of other stable modes, and that a wide variety of small nudges would have similar effects. The article mentions that a study of schizophrenia found that it was characterized in part by extremely rare mutations, suggesting that the brain is at an unstable local maximum, vulnerable to degrading into schizophrenia if unbalanced in any direction. Schizophrenia may in a sense be a much hardier fallback mode of brain operation.
Many of these "disorders" manifest before reproductive age; depression, anxiety, ADHD, bipolar, even schizophrenia occasionally.
Think about the condition we currently call the "disorder" ADHD.
We can easily imagine how the behavioural tendencies ascribed to it would have been beneficial in hunter-gatherer times; i.e., it would be beneficial to the tribe for a certain number of people to be restless and explorative, rather than being conformist and content doing repetitive tasks. Same goes for bipolar; think about how many of our leading writers, inventors, musicians are diagnosed bipolar, and how useful it would always have been to have had a few people with that kind of creativity and propensity for bursts of novel productivity in the tribe.
And I can easily think of ways depression and anxiety are adaptive and supportive of survival, rather than not (for depression it's the ability to contemplate/reform a troubled life, and anxiety it's the avoidance of danger). Even schizophrenia in certain circumstances.
And of course when we invoke Occam's razor, it makes far more sense that these genes are there because they're beneficial, rather than being there due to a historical accident.
And the fact that they're widespread means they're selected for, not just not selected out.