This is true. But did you actually read what the study said? They point out that there is a small number of referees who consistently have their own work cited

This is true. But did you actually read what the study said? They point out that there is a small number of referees who consistently have their own work cited at a much higher rate than the majority of the referees. If referees and authors are semi-randomly matched, there's no reason a small number of referees should consistently get to review papers that have incorrectly omitted references to their own papers.