The WHO screwed up for systemic reasons that commentators, at the time and without the benefit of hindsight, were able to call out. Specifically, they repeatedly claimed there was "no evidence" for cloth masks and "no evidence" for aerosol transmission. Absence of evidence is not evidence of absence, and the organisation meant to coordinate a global medical response should know that. Many respiratory viruses spread by aerosol transmission, and it would have been responsible to say "we do not have enough evidence to say how COVID spreads yet." Additionally, there was evidence for the efficacy of cloth masks, but the papers were studying whether cloth masks were adequate replacements for surgical masks and N95 respirators in hospital settings, and they (rightly) concluded that cloth masks weren't as effective as actual PPE, from which the WHO (wrongly) concluded that cloth masks weren't effective at all.