Could it be that the molecules of 'fart' you are smelling are far smaller than than droplets potentially carrying covid?If they're both wearing pants and fabric masks they can still smell each others farts.
Last time I was in a mall I was in a store handling some items. I gelled my hands after. In the mall I passed a washroom so I though I'd wash my hands too. I was wearing a blue disposable mask. Washing my hands some dude was in a stall taking a shit and I could smell it the whole time I was in there washing my hands.
After I thought if I can smell that guys shit (which is microscopic airborne shit particulate) what's this mask doing to protect me from a virus or someone else if I was exhaling?
I do love that our discussion of science has devolved to 6 year old levels.