I'm sorry, but we've known that masks work for literally a century. They were also recommended during the 1918 pandemic. There is a wealth of literature on this topic; it is scientific consensus.

It's great that you're thinking critically about the ways that this individual study might fall short, but trying to poke holes in a study here or there does not undo the (literally) century of research underlying the efficacy of cloth masks in preventing the spread of airborne illnesses (or epidemiology in general).