The article isn't citing other studies in the traditional sense of a meta-analysis (which maybe you didn't mean). It's four studies conducted by the author whose results are self-contained within this paper and effectively function as a pipeline: study 3 relies on the results from study 2 relies on study 1.

I didn't at a glance see study 4 in the preprint, but maybe that's only in the official print version. I plan to read the methodology in full later.