This article is a demonstration of a method of mask testing. It's not a test of masks. From the authors "As the title indicates, our work focused on developing a simple technique for mask evaluation that can be replicated at other labs, rather than a comprehensive mask test.

They do not test how long the masks stop droplet transmission. Other studies say that these masks start losing effectiveness as soon as you put them on, and most masks are ineffective after a few hours.

And, again, we do not care if masks stop droplet emission. (It's very likely that they do). What we care about is whether the public wearing masks in a pandemic stops virus transmission, and at the moment we don't know if it does or not.