I'm not sure what difference that would make since masks are already mandated everywhere by state and local authorities. The real issue with masks is that (a) many people don't wear them properly (I routinely see people with it only over their mouth, not their nose, or with a huge gap between their face and the mask) and (b) anything that covers your face qualifies as a "mask", no matter how coarse the weave of whatever material it is made of.
So the only argument I see otherwise is that there's some radical difference in the R number between the viruses such that masks only lower substantially enough for flu and not SARS-CoV-2. But that's a much more specific argument that I don't see being argued much less substantiated and proven.
The flu viruses are a different class of virus (rhinovirus) from SARS-CoV-2 (coronavirus) and could be expected to be affected differently by mitigation efforts. I don't know that the much lower flu numbers tell us that people are wearing masks properly; they could be due simply to social distancing and people staying home. Also, AFAIK there is not significant evidence of transmission of flu by asymptomatic people, whereas there is for COVID-19, so it is easier for people to avoid transmitting flu by self-isolating if they have symptoms.