I just had a quick scan of the article, I am going to have to do a bit more of an in-depth read and side research, but one thing that jumped out for me is if the confounding factors included things like accidents.
As insomnia increase the risk of accidents and people who take sleeping tablets obviously have insomnia, but I would also think that being under the influence of sleeping tablets might also increase the risk of accidents.
This may have been included in the study, but I didn't see it specifically mentioned.