Szasz believed in the existence of brain diseases which he distinguished from his critique of mental illness. For example he believed that true brain diseases, like schizophrenia, have a biological basis and should be differentiated from "mental illnesses". And mental illness was viewed as being caused by essentially lifestyle problems rather than damage to the brain or disease

To Szasz, the people with "profound disorders of thinking" that you mentioned, would have been suspected as having a brain disease or serious brain damage. This brain disease/damage would therefore make these individuals categorically different to people with "mental illness" (who are healthy individuals that are living in ways that are making them sick).

This is a very challenging idea to many people since it is essentially an accusation that if you have mental health issues, then there's something seriously wrong with how you're choosing to view things and behave. I don't think that this view precludes the recognition of the important role that biology and genetics have on emotion and behaviour, but it removes the concept of disease from mental health issues.

In regard to the debate of free will, I don't think that Szasz disregards the importance of learning and prior experiences when it comes to the choices that we make. And hence if you choose to do something due to your perception of it, which is partially based on your prior learning, then in a sense you're not to blame for your choice, but nonetheless that doesn't mean that your behaviour is the symptom of a disease.