Was it a double blind study?
This is not true. It's only one kind of study.
In this case, the study was looking for a correlation between diet and cancer diagnoses. All it needs to be valid is to conform to the scientific method. Placebo-controlled therapeutic trials are not the only kinds of studies that conform to the scientific method, and indeed not all of them do.
Please try to avoid reflexively invoking this trope to unfairly dismiss potentially interesting studies.
Meanwhile, have a bit to read on the ethics of placebo-controlled clinical trials (which is what is meant with "double blind" study here): https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3844122
https://jamanetwork.com/journals/jamainternalmedicine/fullar...
>> Design, Setting, and Participants In this population-based prospective cohort study among French adult volunteers, data were included from participants with available information on organic food consumption frequency and dietary intake. For 16 products, participants reported their consumption frequency of labeled organic foods (never, occasionally, or most of the time).