The study was designed to tease out mask-wearing by attempting to control for other factors via its study design. — They did this specifically by comparing a ma

The study was designed to tease out mask-wearing by attempting to control for other factors via its study design. — They did this specifically by comparing a mask-wearing city with a synthetic control unit based on reference populations without mask-wearing rules. So the hypothesis “mask-wearing helps” is, by design, not post-hoc. You’re right that it’s a single data point (though with a strong effect size). And, being an observational study, it simply can’t exclude all confounding factors, and will never approach the strength that a controlled study could. Nobody is disputing that. Interpretation of observational studies always needs to be tempered by the reality that it’s impossible to control for all confounders. But within these given constraints the study essentially provides as strong evidence as could reasonably be expected (unless the study has a flaw that I’m overlooking and which, as far as I know, hasn’t yet been pointed out elsewhere). Their design of the synthetic control is statistically sound and controls for a very large number of reasonable confounders. And, contrary to the claim in the comment I’m replying to, data from other cities does not invalidate the study’s findings. As far as I can see, the study’s inadequacies are all due to external circumstances and not because it’s a bad study.