So here's the problem. Homemade masks are somewhat effective because they stop people who are sick from spreading some droplets around. But the second someone with a mask scratches their nose, adjusts their mask, takes their mask off, etc., they have transferred the virus from their mask to their fingers and from their fingers to literally everything they touch. This study looked only at air samples, which captures only a small slice of possible transmissions. Another issue is that the SARS-CoV-2 virus is quite small, so the efficacy of a homemade mask may vary significantly depending on the materials you use.

For the general public, masks are meant to stop sick people from infecting others, not the other way around. This study looked for the virus in air from someone breathing out. They didn't look at what you might catch if you breathed in through the mask. In a pandemic situation, the only way to guarantee that you haven't spread the virus around is to stay home if you have respiratory symptoms or if you are at risk of exposure (travel, close contact with someone who is sick). You can't game public health.

Here's the bottom line. Lots of people running around with dirty masks pretending that they're safe means: we have to deal with social distancing for a longer period of time, more people are affected by loss of income, more people get sick, and more people die. Just stay home. This is probably the only time you can help by sitting on your ass and doing nothing.