“Airborne” and “aerosols” are not synonyms. You’re right about aerosols but the question above is whether we knew that the virus traveled in a manner that a mask could interfere with. That includes short-range droplets, not just aerosols.
We knew very early on that droplets were likely and that masks could block many droplets. The question was whether the mask would itself become a major vector in the process.
No, the question was whether it was droplet or aerosol. Experts insisted on droplet even though evidence pointed at aerosol, and cloth and surgical masks don't block aerosols, the particles are too small. That's what people are referring to when they say "masks don't work".
We knew very early on that droplets were likely and that masks could block many droplets. The question was whether the mask would itself become a major vector in the process.