> I’m not sure what other confounding factor is powerful enough to counteract hundreds of people in close proximity breathing the same air if cloth masks have an insignificant effect.

The difference in populations. If your population is younger, fitter, lighter skinned (and that's just three of the most important of the many factors found in severity of the disease) then there could be the same number of cases in both populations but yours has a higher number of asymptomatic or symptomatic but self-misdiagnosed cases.

Unless both populations are being mass tested weekly? Or do they only get a test if they show symptoms…