I think you may be misinterpreting the point of the analysis. They were studying the "Lessebo" effect more so than the placebo effect. Basically, Lessebo is sort of the opposite of Placebo. Essentially, the argument has been made that patients in PD studies which have a placebo group may experience a negative expectation of benefit due to the possibility getting the placebo. The net effect, according to the Lessebo theory, is that patients getting the active drug will change less when in a placebo controlled trial as opposed to when there is no chance of getting placebo. Lessebo reduces the recorded effect of and active ingredient. Placebo increases the effect of an inactive ingredient.

There have now been several studies where results showed that having a placebo arm reduces the measured efficacy of an active treatment significantly. The result would mean that a drug may have a larger magnitude of effect than that which was measured in the trial.

Tiago Mestre is a leading proponent of this theory. The meta analysis you linked is his latest attempt to prove that Lessebo exists, that a placebo controlled study will reduce the magnitude of change after drug treatment for PD patients.