A quote: "Recent scholarly and media accounts paint a portrait of unhappy parents who find remarkably little joy in taking care of their children, but the scientific basis for these claims remains inconclusive."
Yes, and this study suffers from the same problem -- insufficient rigor, obvious bias on the part of the investigators, no control group, sloppy experimental design. A typical psychology study that discovers exactly what the researchers expected to discover.
A meta-analysis of all the studies on this topic would produce nothing that would contradict the null hypothesis (of no significant measured effect), as has recently happened with respect to depression medications.