This is a common misconception about the review. It is true that none of the studies they looked at were double-blinded but they were still included if they were designed and conducted well enough. In a Q&A shortly after the review's release Cass demonstrates that she is well aware that exclusion based on this would be silly.
The amount of myths circulating about the review prompted the publishing of an FAQ page which deals with some of the more egregious examples (e.g. the claim that 98% of studies were rejected).
There was a (successful) effort to push misinformation regarding the report.