As someone with experience in this field (and plenty of skepticism of geographic association studies), I find the article lacking. Is there a technical write-up?

It is quite odd that the authors say that they don’t know if the original method can yield negative results. Some of the results are expressed as relative risk, so there’s a numerator and denominator. That should be enough to begin to work it out, since all possible exclusive numerators sum to the denominator. It feels like if you don’t understand whether some areas will be lower than average, then perhaps you don’t understand the method sufficiently.

It sounds like they reverse engineered the original method, but that means they could be applying an overfit model and getting spurious results that the original wouldn’t give.

That’s all speculation, hence we need the technical write up.