I would have to read the actual study to determine, but in general, a case report—that's not a weakness. Part of the problem you have with a case report is that we're dealing with a distribution of people who get illnesses or don't get illnesses.
The best analogy I have is that if I had a jarful of multicoloured M&M's, a case report is like reaching in once, pulling out a red one, and saying they're all red. You need to pull out more M&M's to get a good understanding of what the distribution actually is in the bottle.
That's what these studies are intended to do.