In exactly the same way that recirculating air from inside the vehicle is easier on your air-conditioner, recirculating interior air is easier on the filter.

With the A/C, you want some minimum outside air to get rid of CO2 and whatever other effluents the passengers and materials in the car are generating. But if you were driving through a plume of toxic gas/particles, you wouldn't mind smelling your armpits and a little extra CO2 if it meant extra protection from whatever is out there.

Hence I believe that if you were really designing a Bioweapon Defense Mode, you would cut the intake of outside air to an absolute minimum. You might not go for zero flow, because you might want to pressurize the cabin a bit (in the U.S., most building mechanical systems are set up this way). But I wouldn't expect to cycle through enough outside air that you could halve the concentration in that bubble (which is probably 10-20 car volumes or more) in less than 15 minutes, as their graph shows.