They don't fluoridate the water here in Portland, OR. Compared to other parts of the country where I've lived, the difference in people's teeth is pretty shocking. I haven't heard any alternative hypothesis that would explain this difference better than the lack of fluoridation here.
As I understand the article, they're saying that they couldn't find rigorous studies demonstrating the beneficial effects of fluoride. They don't claim there's any evidence against fluoride having beneficial effects, just that the studies haven't been done.
With over half of people currently in the Portland metro area having been born elsewhere, it would be hard to sort out whether cavities were the result of fluoride deficiency or some sort of selection bias in who chooses to live there. I never noticed a lot of fillings when I lived in Portland, but then I wasn't looking inside people's mouths.
There's too much inertia in favor of fluoride being beneficial for any serious researcher to risk their credibility. Everyone "knows" fluoride makes teeth stronger, like lettuce being healthy to eat, or global warming is human-caused.
In fact, even discussing the possibility is too much for most people to entertain. They think you're wasting their time with fantasy and falsehood, like conspiracy theorists always do. Fluoride being bad for teeth is as believable as airliners dropping chemtrails everywhere, or jet fuel not "spheroiding" steel after hours of heating.
See? Even posting about the idea of researching it merits a downvote. The idea that fluoride has no benefits is as popular as Apple and Microsoft merging.
As I understand the article, they're saying that they couldn't find rigorous studies demonstrating the beneficial effects of fluoride. They don't claim there's any evidence against fluoride having beneficial effects, just that the studies haven't been done.