Maybe I'm missing it, but it doesn't seem to show anything that says that it was masks that prevented Covid spread? There wasn't a non-mask control (unsurprisingly, in an unplanned experiment), and we don't have a good comparison for how high the rate of transmission is expected to be in similar conditions without masks.
None of which means masks _didn't_ help, but I don't see anything in here that shows evidence that masks _did_ help. Unless there's a comparison here that I'm missing?