You’re right, this is just a summary of the existing evidence, not even a systematic review. They list their “top five” studies with no indication as to how/why they were selected (let me guess - they all found positive findings for mask wearing). When you say there are studies citing the same studies, do you mean there are multiple studies based on the same dataset? It wouldn’t be unusual for studies on the same topic to cite the same literature, as the intro usually provides a summary of existing evidence, and findings are then compared with previous studies in the discussion.