They summarize their positions clearly in the introduction which is basically on objective observation the placebo effect doesn’t emerge in the physiology but in the self reported effects. While they seem to hold psychology as not relevant because it’s not objectively observable, the less hard ass way to read this is the placebo effect doesn’t produce objectively measurable effects on physiology but it does appear to have a psychological effect.
They are positing the psychological effects are confirmation bias and conforming to expectations, but what they don’t address is whether the psychological outcome is real. Depression is greatly impacted by our psychology and whether the effect is due to some social conformity, beliefs, or other non physiological basis, is irrelevant to the efficacy.
Ultimately the authors argument is based on the notion that psychology is not real science, because science deals with things you can directly observe, and anything that isn’t real science is false. This is obviously fallacious. Even assuming psychology isn’t a real science, and psychology researchers have done themselves no favors here, the second statement is where the fallacy lies. Well done science is generally not false. But not everything that is true has been explained by science. Something can be true without us knowing the facts of how it works. Psychology requires self reporting by its very nature and as such is subject to all sorts of bias and difficult to control aspects - some impossible to control - and it’s likely some aspects of psychology are cultural and contextual and change with time. This doesn’t make it not real and not true. It just makes it difficult.