I don't agree with that statement. Even if it only blocks 1/2 of the viral load you are better off. Best case: you don't get any of the virus, worst case: you get a smaller dose which helps your odds.
That means covering the mouth is most important because even breathing releases droplets. And the nose should be covered too, but only covering the mouth does help.
"This letter responds to your question concerning the possibility that [coronavirus] could be spread by conversation, in addition to sneeze/cough-induced droplets," the letter states. "Currently available research supports the possibility that [coronavirus] could be spread via bioaerosols generated directly by patients' exhalation," it continues.
This article offers some links and data on the effectiveness of even homemade masks at filtering particles:
https://smartairfilters.com/en/blog/diy-homemade-mask-protec...
https://www.researchgate.net/publication/258525804_Testing_t...
"Guaranteed protection" does not matter at all.
Masks that only offer 99% protection are well worth wearing. As are 40% masks.
https://www.independent.co.uk/news/world/americas/coronaviru...
Agree that covering the mouth is the most important thing. Hopefully this person just had it down while he was outside and not around others.
This is the same reason the medical authorities have recommended these types of masks for healthcare professionals in areas where N95's are not available. It's better than nothing.
https://www.cdc.gov/coronavirus/2019-ncov/hcp/ppe-strategy/f... (see "HCP use of homemade masks")
"even a properly fitted N95 respirator does not completely eliminate the risk of illness or death"
https://www.fda.gov/medical-devices/personal-protective-equi...
Probably not calibrated on Coronavirus transmission.
https://www.cnn.com/2020/04/02/health/aerosol-coronavirus-sp...
https://www.independent.co.uk/news/world/americas/coronaviru...
To categorically say it always makes more harm that good seems entirely wrong!