It’s actually exactly in line with what the link above says.

In June 2015, the Cochrane Collaboration—a global independent network of researchers and health care professionals known for rigorous scientific reviews of public health policies—published an analysis of 20 key studies on water fluoridation. They found that while water fluoridation is effective at reducing tooth decay among children, “no studies that aimed to determine the effectiveness of water fluoridation for preventing caries [cavities] in adults met the review’s inclusion criteria.”

In other words, water fluoridation might not make much difference for adults, but it can for children.