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The much more nuanced reality of what leaders in that field were thinking (from an epidemiologist):

Epidemiology's Time of Need: COVID-19 Calls for Epidemic-Related Economics - American Economic Association

In January 2020, there was strong evidence supporting the use of personal protective equipment, including face masks, in high-risk settings such as health care facilities for the prevention of respiratory infections. However, the existing epidemiologic literature on the use of face masks by the general public for control of respiratory infections was extremely limited and showed mixed results (Brosseau 2020; Brosseau and Sietsema 2020; Chu et al. 2020). For example, one meta-analysis found that mask use in health care approximately halved the risk of influenza infection (Saunders-Hastings et al. 2017), and a randomized trial of non-pharmaceutical interventions in the home found an approximately 20 percent reduction in influenza infection for households using both face masks and hand sanitizer compared to hand sanitizer alone (Larson et al. 2010). In contrast, several randomized trials of households limited to face mask use alone had found no reduction in influenza transmission (Aiello et al. 2010; Canini et al. 2010; Cowling et al. 2008).

Lacking clear information on the benefits of community-level face mask use,epidemiologists in early 2020 engaged in internal discussion about the potential harms and benefits of this intervention, considering aspects such as the limited existing research, the limited supply and interrupted supply chains of masks, what was known at the time about the epidemiology of SARS-CoV-2 transmission, and concerns around the potential for “risk compensation” if people who were wearing masks then engaged in fewer other preventive measures (Bamber and Christmas 2020; Brosseau 2020; Brosseau and Sietsema 2020; Cheng 2020; Javid, Weekes, and Matheson 2020; King 2020). Based on these discussions, many applied epidemiologists, including those at the World Health Organization and Centers for Disease Control, initially advised against the use of face masks by the general public. Instead, they stressed the importance of hygiene and distancing-based interventions, such as hand-washing, social distancing, and quarantine.

Over time, however, new information emerged. First, it became clear that at least some subset of Americans would be amenable to wearing masks. Second, we learned that SARS-CoV-2 could be transmitted by individuals who were not (yet) symptomatic (Gandhi, Yokoe, and Havlir 2020). Finally, as the availability and use of both fabric and surgical masks increased, it became clear that even when individuals wearing masks did increase their risk behaviors (by, for example, joining protests), the evidence did not suggest that transmission in these settings was any higher than if attendees had been unmasked (Dave et al. 2020). Together, these observations have shifted most applied epidemiologists and public health officials towards encouraging the use of face masks by all individuals (Greenhalgh et al. 2020; Roderick et al. 2020).

However, this recommendation does not mean that the academic epidemiology of face mask usage by the general public during respiratory outbreaks has necessarily advanced much beyond what we knew in January 2020, and many academic epidemiologists remain agnostic about the value of face masks. In fact, if anything, it may be fair to say that academic epidemiologists have fewer answers about the science of face masks than we did 10 months ago—simply because we now have more questions.

Previous research on face mask usage in respiratory outbreaks focused chiefly on evaluating either N95 masks or surgical masks, both of which are subject to regulatory standards. In contrast, many of the face masks used by the general public during the COVID-19 pandemic are made from fabric, both commercially and homemade, and the filtration efficacy of these masks is both unknown and potentially highly variable (Aydin et al. 2020; Davies et al. 2013; Tcharkhtchi et al. 2020). In addition, previous studies of face mask usage typically assumed individuals had been provided with training and guidance on how to appropriately don, doff, and wear face masks to maximize their benefits. In reality, adherence both in terms of frequency and correctness of face mask use is extremely variable among the general public. Despite this, existing attempts to model the population impacts of community-level face mask use have typically assumed perfect adherence and correct usage (Ferguson et al. 2020). Academic epidemiologists likely will be investigating and debating these topics for many years to come, both to fully characterize the causal effect of community level mask-wearing strategies and to explore the actual risks and benefits that result from these (Bundgaard et al. 2020; Doung-ngern et al. 2020).