Exhaling through a sieve still blocks the largest wettest particles, and limits the distance virus is forcefully expelled through the air. Again, it's not perfect, but infection is purely a numbers game. The more virus you can deposit on a mask, or a sleeve, or even a sieve, the less virus is in the air and the longer it takes for a virus to go from a mouth on one side of a room to a receptor on the other side of a room the better.
