>excluding diagnoises of cancer that occured outside of the town even if the diagnosises were on town residents.
I would be floored if they did this. I'm a statistician who reports on cancer rates for another state, and the universal standard for assigning locations to cases is the patient's residential address. Looking at Georgia's 2016 report[1], they mention using the SEER*Stat software provided by SEER[2]. I have the same software, and I've never seen it offer any information about the reporting facility beyond a general category ("hospital", "lab", etc). They would've had to work outside the well-worn paths using a custom extract from the cancer registry. Not impossible, but also not reasonable.
It's true that epidemiologists, or whoever looked into the clusters, could've been a separate unit from the statisticians behind the annual reports. And maybe they're not used to cancer epidemiology and didn't ask about best practices. But I'd be surprised if they messed up this basic thing.
Also, I've heard of the study mentioned in the article where only one out of 428 cluster investigations proved true. But the way it was portrayed at a cancer registry conference, the low rate is a combination of small sample sizes and over-reporting of "clusters." Cancer's a rare and frightening event, which are two things people have trouble seeing rationally.
[1] https://dph.georgia.gov/cancer-reports
[2]https://seer.cancer.gov/