Seems legitimate, though that doesn’t mean it’s a particularly valuable analysis.
> If it’s trusted, does that mean that our efforts to mask were basically a waste of time, or not?
It doesn't say.
In a world where everyone wakes up each day, checks the daily cases nearby, and only puts on a mask if it’s over 1000… you'd get near perfect correlation between mask use & covid cases, even if the masks prevent every single transmission.
And that’s not all that out of line with reality.
High rates of Covid absolutely cause increased levels of mask compliance, so seeing a correlation between the two doesn't really tell us much about mask effectiveness.
You could maybe do some time-series analysis to try and get a better idea. The decision to mask today would be influenced by cases in the recent past, and any actual effect of mask wearing would only affect diagnoses 3-5 days later. But I’ve not done that math, and neither has this paper.