frongt noted that the fluoride is beneficial (for preventing tooth decay), while you state that the study denies this. This is untrue, as it just shows that fluoride doesn't affect IQ (which primarily focuses on measuring logical thinking). It does not look at how it protects against cavities, there are plenty of studies on that already!

It’s the same thing as if you tried to give an IQ test to someone who, in the past, has had a bacterial infection. Then, when the person is perfectly healthy, you give them an IQ test once, then some antibiotics, then another IQ test giving the same result as the first. You would not conclude that antibiotics are ineffective and should be banned!

Your argument would replace bacterial infection with cavities and antibiotics with fluoridised water. Like the example with antibiotics, it is not a reasonable conclusion to state that no change to IQ = fluoridation in water is ineffective.