I believe the reasoning is that aerosol would have ultimately spread everywhere, including on the AC itself, while ballistics can be heavily influenced by airflow, but are still limited in range.
Then again, it's not aerosolized as it didn't diffuse homogenously into the space. It's somewhere in between.
When these guys are saying "ballistic", they mean that the droplet is big enough that gravity is important, and that it's going to hit the ground in a reasonable amount of time and not hang around in the air until convection/diffusion spread it evenly throughout the room.