Retributivists would support a not guilty plea for persons who are in fact "insane", because of their mental dysfunction, they then are not responsible for what they do.

This is, in fact, true. While Retributivists are those who support the idea that the punishment of criminals be in line with the crime they have committed, they also believe those those who are mentally ill, or legally and medically insane, have a mental dysfunction and cannot be held accountable for their actions.