I think many people are conflating two related but separate issues.
The fact is the Covid19 virus (all viruses for that matter) are smaller than the “weave pores” of ANY filtered mask; be that cloth, surgical mesh, or even the N95. Can and will these masks stop some virus transmission? Yes, just as a HVAC filter stops some dust from entering your HVAC duct but not all dust particles. Even the dust particles that are smaller than the pores in the HVAC filter can get caught on the fiberglass strands in the filter. Just as some of the virus can and will get caught on the weave of the mask. However, most Covid19 virus would easily penetrate even the best mask.
Here’s the thing though, the consequential amount of airborne virus transmission between people is carried from person to person on spittle droplets. Spittle droplets may contain thousands if not millions of virus because they are exponentially larger than the virus. These spittle droplets are also larger than the “weave pores” of almost all masks. Meaning that when you exhale them into a mask the overwhelming majority of the spittle and virus will be captured on the mask.
That is why the answer to, will masks stop the spread of the virus, is not a definitive yes or no answer. Individual virus can easily pass through even the best filtered mask. However, spittle droplets, the main form of transmission, are too big to pass through most masks.