> Due to the non-random sample, as well as the lack of placebo control, it could be argued that sub-acute enhancements are due to uncontrolled factors such as psychological expectations, or the environment in which the drug is taken.

Indeed. The setting of the retreat (“After ingestion, participants were instructed to stay on the premises, and were able to do what they wanted, as long as they did not disturb other participants. Facilitators provided music, tools to draw and/or write, and food. In the evening, all participants and facilitators came back together as a group.”) might have had a significant effect on the results. These should be, at least, compared to a similar retreat where no psilocybin is ingested.

Otherwise, the effects could come solely from the environment, e.g., a calm and relaxing retreat far from a busy life.