People say this but the obvious point is that flu numbers are much lower if not vanishingly small, suggesting people are in fact largely complying with mitigation efforts.

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.