The website wasn't able to test the mode against actual bioweapons (thankfully), so we still don't know if it works as Tesla claims, but it does effectively remove foul odors from the air as it enters the car.
What actually happens when "Bioweapon Defense Mode" is engaged? In short, nothing.
Per Business Insider:
"…we wanted to see what would happen when we engaged the feature."
"But … would alarms sound? Would lights start flashing, like they do when the Starship Enterprise goes on red alert? Would the voice of the navigation system say "Bioweapon Defense Mode engaged!"
"Actually, none of that stuff happened."
"In fact, absolutely nothing occurred. Total silence."