I agree, someone in psychosis at the time they commit a crime, may be found not criminally responsible.
The issue to be decided in court is whether they were psychotic at the time of the crime. It is not enough of a defense, IMO, to just have evidence that they had schizophrenia, or were depressed, or were suicidal, etc. Actual psychosis has a number of markers, there is usually evidence of disordered thinking, brought in as evidence.
Similarly, IMO, it is not enough to say a women was experiencing post-partum depression, to automatically assume she was psychotic at the time of strangling three children.
What I think people are really saying is 'she must have been psychotic, she wouldn't have done it otherwise', but I don't think that's a valid legal argument. People used to say that about fathers who killed their family, or children who killed their parents. "They must have been insane", although the word they actually used was 'mad'.
JMO