That aside, in particular, was meant in the sense of: The mental illness can be seen as causal or mitigating. Like, a person suffering from a schizophrenic break could reasonably use that as a defense (not to avoid all treatment/punishment, but as a mitigating factor or to have the consequences moved from prison time to a medical facility) for a crime committed during that break (a sincere belief that they're in danger or that someone was the devil leading them to commit an assault or worse). Depression could similarly be used in a defense like that as depression (in severe cases) can similarly cause breaks with reality or reduced capacity for self-control.

But having severe depression or being schizophrenic, on its own, is not a good justification for changing/reducing charges and consequences (sadly, consequences in the US are punishments as there's little focus on rehabilitation). If anything, it could reasonably affect the sentencing to account for the riskier consequences of placing a person with that mental condition into a medium or high security prison or general population, but not the selection of charges by the prosecutor.