This news coverage appears to completely miss the primary point of the study's findings.   Hexafluorosilicic acid, the overwhelmingly most common fluoride water additive was found to protect against diabetes.  

Quoting:  "Among the three used in this data set, fluorosilicic acid was most significantly associated with reduced diabetes incidence (β ¼ –0.45, P< 0.001), inducing the greatest decline in incidence apart from the year variable."

So the study's mathematical model found the most common additive affords protection from diabetes with a statistical uncertainty of under 1 in a 1,000.  

From the perspective of an oral health advocate, I'm not upset if a water district chooses the more common, less expensive water additive rather than sodium fluoride. . just fine with me.