https://onlinelibrary.wiley.com/doi/full/10.1111/cdoe.12215
Perhaps more could be done. The situation is complex because of several compounding factors for sure. There are European countries that have no water fluoridation and better oral health outcomes than in North America.
Regardless, there’s 10 years where a city in North America turned off water fluoridation and we have results of that decision to study.
"For all tooth surfaces among permanent teeth (Table 1a), there was a statistically significant decrease in Calgary, for the overall mean DMFS, which was not observed in Edmonton."
Based on their data, you could argue that fluoride increases cavities in adults... I'm not making that argument. I agree with you in that I think confounders are at play and the difference attributed to fluorinated water isn't as large.
People will use this study to take about the rampant tooth decay in Calgary, ignoring that there is roughly as much decay in Edmonton which had the fluoride on the whole time.