The people that want to make criticisms about how this association is not causal can start reading halfway through the first column of page 1252. The authors are careful to phrase their finding as an association, not a causal relationship.

They discuss the confounds you might think of (sleep a lot because of undetected major illness? depression? etc) and their efforts at controlling for them.

They then acknowledge that there are still enough unknowns that they can't make a causal link, and that there should be more in depth and controlled experiments to investigate this phenomenon.