I find this:
However, last February, the European Food Standards Authority (EFSA) refused to approve the statement.
A meeting of 21 scientists in Parma, Italy, concluded that reduced water content in the body was a symptom of dehydration and not something that drinking water could subsequently control.
Now the EFSA verdict has been turned into an EU directive which was issued on Wednesday.
... which certainly supports your claim
A meeting of 21 scientists in Parma, Italy
bowel function after looking at three studies of prune consumption.