I have a bunch of dust masks around for when I am doing woodwork and making a lot of fine sawdust, sanding off drywall mud, using a grinder. That is what the masks that I have are designed for. They are not designed to stop coronavirus particles that are somewhere in the neighborhood of a thousand times smaller in diameter than the particles of dust I am making.
IOW, your masks are for YOUR protection.
They’re designed to stop your spit when you cough or sneeze.
The statement is accurate, but doesn't fully describe the situation. Virus "particles" are very small, but are not the complete problem. They are carried on droplets of mucous and saliva from an infected person, which are much bigger. Dust masks can stop these larger droplets. Viruses floating around in free air are generally not the problem.