The paper doesn't actually say that. It says nobody really knows basic things you'd need to determine that, for example:

Given how little is known about the production and airborne behavior of infectious respiratory droplets, it is difficult to define a safe distance for social distancing

There are plenty of scientists saying masks have no effect when worn by people who aren't actually both visibly sick and working in healthcare:

https://www.cidrap.umn.edu/news-perspective/2020/04/commenta...

In fact manufacturers of ear loop/cloth masks specifically say they can't stop COVID:

https://off-guardian.org/wp-content/medialibrary/EYZu2Q2X0AA...

The WHO said:

https://www.foxnews.com/world/who-guidance-healthy-people-we...

"If you do not have any respiratory symptoms such as fever, cough or runny nose, you do not need to wear a mask," Dr. April Baller, a public health specialist for the WHO, says in a video on the world health body's website posted in March. "Masks should only be used by health care workers, caretakers or by people who are sick with symptoms of fever and cough."

There's plenty of contradictory research out there. Maybe a mask helps if you're literally coughing up phlegm all over the place, but COVID has a dry cough and obviously symptomatic people don't seem to be the only way it spreads.

And fortunately, it's easy to understand the mechanism of action that explains the positive impact of masks: the primary means of covid spread is virus conveyed not individually but in large water droplets. Any filter that impedes the flow of these droplets is very likely to diminish the spread of the contagion. Therefore, anyone who is physically within the field of oral spray of an infected person should benefit from BOTH parties wearing masks.

If all scientific explanations also came with strong and plausible proposals of causation, science-based advice would be much harder to dismiss, whether that dismissal is motivated by mechanism-free (and evidence-free) theories like tribalism or not.

This is a good source. I'm baffled by people still asking for "evidence" of mask effectiveness despite tons of proof for it.

Something tells me this comes from people who cannot be satisfied with rational/scientific proof anyway.