Yes, this is reasonable, in the general case, absent a specific example to examine. However, masks do in fact work to prevent the spread of certain kinds of disease, based on both obvious mechanistic reasoning and on actual experimental evidence to that effect. Making the affirmative claim that masks would not work against covid (at the time) would have been ignoring or denying the non-trivial possibility that they would work quite well (or work poorly, but working poorly is still working on the margin).