I think the problem people see is more with creating an equivalence of political opinion and mental health.
The full quote about anti-capitalism doesn't say anything about what makes someone an extremist. It doesn't say anything about rational criticisms and irrational ones it just relates political attitudes. It doesn't say anything about how we would separate rational criticism from extremism. That is a problem, no?
The take home message doesn't help at all when a dissident deals with an oppressive system. E.g. how would that message be applied in Germany in 1933? Or 1939? Or 1942? I don't think it can at all. How would it be applied to say US intervention, or colonisation? Again, I don't think the message would hold up.
Do you think that the scientific method is applied here in an appropriate way?