I think that both the mentally ill, and the mentally healthy, can have the potential to say things that shouldn't be said. However, I seem to spend a lot of time reading about psychology myself, and realize that by doing that, that there are mountains of words out there trying sculpt out what those things even mean
Presently for example, I am reading about the positions of augustine as contrasted by that of pelagius - not psychology, but wait a moment - where do you start searching for real objectivity in what either of these characters have said or done?
Take augustine for example. Am I take it for granted as being sane, that a man would stand before an audience, or before a blank scroll, and write out, or verbally express, the uncertain salvation of an infant? It's important, because he would influence a lot of people across time.
I might declare, or not even declare - merely take it for granted, that the man and his actions and thoughts are sane. And, given the religious freedom which our country gives us, I would, if I believed in what he said, think that he led a fully sane and functional life, and that I am sane and functional for presently sharing beliefs he had
Or, I might appeal to what I think is objective (a wrong view, depending on one's religious choice), and declare that if a man spent a mere two minutes of his life, in thinking that the salvation of an infant is uncertain, that this would be an absolutely unproductive, and even insane use of time and energy. Likely, that is what pelagius thought of augustine. And augustine probably thought the same of pelagius - and down through history, right to our time, people have had wildly different perceptions and views, originating from unique and individual minds
So I guess my post is just to show the context that might be required behind the statement in your post - the fact that there has been an extremely wide latitude between views, and how that has to enter the equation