I can't buy it, European citizens almost all have to wear masks and we can clearly see that this measure alone doesn't seem to have an impact on the epidemic as the speed of the increase of cases is more or less the same that what we had in march. And nobody was wearing masks at that time.
That is a good point which is worth discussing. From an opposite point of view Czech Republic mandated masks early on which seemed to work. Now they are in a bad state which is being blamed on relaxing the restrictions they had in place.
Hypothesis: Wearing a mask is a physical reminder that we're in a pandemic, which helps folks remember to do other things which reduce their risk (avoid touching things unnecessarily, wash hands more often, staying at home unless necessary, etc)