Well, first of all. They are not demonstrating causality with this study because the only scientific method to do that is through multiple confirming double blind randomized controlled trials ("RCT"). That's sort of my point in my previous posts.

If there is a method to test causality of the placebo effect, that's the only way to go about it. (I'm a statistician, btw, that's why I believe this to be the case.)

The placebo in RCTs isn't given to prove the placebo effect exists. It's given to create a control group on the presumption that the placebo effect may exist. It's given to account for the possibility of a placebo effect.

I'm not really sure how to test the causality of the placebo effect but, i would point out that there really isn't any reason to presume that there is a singular cause to the placebo effect. And, asking me to do that in relation to this study is shifting the burden of proof. It's not on me to prove that. It's on the authors of this study to demonstrate that they have.