He makes a clearly false comparison near the end by claiming that scientology coerces treatment despite explaining that there is a consent form in advance. Psychiatry requires no consent for forced treatment under any circumstance whatsoever. As terrible as scientology is, psychiatry is a far greater threat.
I don’t think scientology was the main point of his argument. Although I agree with you on one level, rather than being pedantic and calling out every single one of his errors, look at his mmain argument and try to critique that. As he showed by Szasz’s definition of mental illness makes it impossible to have mental illness, and Szazs’s argument is circular. As for scientology I don’t see it as being any better or worse than any other religion.
He completely skips addressing the main issue despite noting it at the beginning. There is no theoretical limit to the range of behaviour that psychiatry can use as excuse for forced treatment. Given that the World Federation for Mental Health still claims to be based on its original principles, there is no reason to suspect that a limit is intended. Steve Wiseman may not subscribe to that doctrine; but, he is not an important person compared to those at the top in the psychiatric associations.
As for forced treatment, I do not agree with it, but it is not a very common thing, and pretty much everything that can be done to prevent it is. Also, you must be deemed an imminent danger to your own life, or the life of others. But this was not really the main point of his argument. The main point he was trying to get across was that mental health is not a myth.