Actually the theory behind the mask is to teach people not to touch their face. Hand to nose, eyes, mouth, yes also spreads the infection.
In Colorado that is the intent as there are not enough masks to mask the entire US population and change on a regular basis.
1 micron = 1 millionth of a meter
1 nm = 1 billionth of a meter = 1 one thousandth of a micron
100 nm = 100 ÷ 10³ micron = one tenth micron
1 nm = 1 micron ÷ 1000
100nm = 1 micron ÷ 1000 × 100 = 1 micron ÷ 10
If your premise is correct, the mask may still stop it because it is usually attached to a larger particle such as a droplet of snot.
https://www.envirosafetyproducts.com/resources/dust-masks-whats-the-difference.html
One thing you have to ask yourself as there is a shortage of PPE including masks, how do people have masks? Everybody here seems to have surgical to N95’s.