There is an argument to made that such data is what matters to evaluate mask mandates. What matters is not what happens when worn properly but rather what happens the way the compliance happens in the real world.
Absolutely. But to frame it as though masks don't work, is false. There are actually people out there who do wear masks properly, and it isn't right to discourage them from doing so.
In fact, this is likely a reason the CDC didn't recommend masks originally, because they felt there wouldn't be much benefit if most people dont wear them properly. Ironically, the same people who harp on that flip-flopping as "proof" that masks don't work, will run around with this study as well, not realizing/ignoring that it is lack of proper usage which was the cause of both. (And even more ironically, in my experience, it's usually the same people who don't wear masks properly, even when they do wear them)
