That study is n=4. They didn't have enough statistical power to make any conclusions of that sort.
Incidentally, if you look at the SK study's actual data, it appears that the cloth and surgical masks reduced the concentration of the virus by about 0.5 to 1.0 log units -- that is, by 66% to 90%. However, those results results were not statistically significant, because n=4.
Nancy Leung, from Benjamin Cowling's lab in Hong Kong, did a much better study, n=114. They found that surgical masks did confer significant protective benefits.
Incidentally, if you look at the SK study's actual data, it appears that the cloth and surgical masks reduced the concentration of the virus by about 0.5 to 1.0 log units -- that is, by 66% to 90%. However, those results results were not statistically significant, because n=4.
Nancy Leung, from Benjamin Cowling's lab in Hong Kong, did a much better study, n=114. They found that surgical masks did confer significant protective benefits.
https://www.nature.com/articles/s41591-020-0843-2.pdf