No??? This is what having a prior is for. In this case, the prior was the mechanistic model which told you with reasonable confidence that masks would work in the slices of worlds where the primary mode of transmission was one mitigated by mask-wearing. We do not in fact live in a state of helplessness absent a double-blind peer-reviewed RCT; you will actively come to incorrect conclusions if you refuse to use your existing models & knowledge of the world to draw conclusions about the likelihoods of various outcomes.
Yes. I'm incorporating a prior. Read what I wrote at the top of the thread: every medical intervention that has ever failed a trial has had a biologically plausible justification for doing the trial. Nearly all trials fail.
In the history of medicine, literally every failed medication, surgery, treatment or intervention has had an explanation that seemed plausible at the time. Just as we're seeing with masks, the vast majority of interventions have little to no effect. Many make things worse. From bloodletting to thalidomide to failed cancer drugs, medicine is littered with examples of people who "knew" that their preferred treatment would work based on "priors" or "plausible mechanisms", and ended up doing great harm.
As a bayesian and someone who is knowledgable of science and medicine, my prior probability of any medical intervention working is almost zero.
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).