Thanks for the link.
I think the authors shared your concern as well.
Our review did not uncover clear causal mechanisms, but the trends in the data are useful for generating hypotheses. It is unlikely that any single causal mechanism explains all the findings in the review. Instead, we hypothesise that a combination of social interaction,61 mindfulness or experiential acceptance,62 increased self-efficacy,33 immersion in green spaces,63 neurobiological mechanisms,64 and acute positive affect65 combine to generate outcomes. Meta-analyses have found each of these factors to be associated with decreases in depressive symptoms, but no single treatment covers all mechanisms.