They did ZERO original research.
What they did was collect earlier studies and merely did a “meta-analysis” - a statistical analysis, of those studies.
What none of the studies have within them is enough on CONTEXT - why, when, how, parent’s emotion, degree of spanking, ect., ect.
A very high proportion of science “studies” today, particularly in the social sciences, has concluded that correlation equals causation - which it doesn’t.