In the Chang article that you posted here on May 7, people at table B were clearly infected by droplets from A1 being blown by the a/c fan over table A and then over table B. The people at table C may have been infected by droplets that managed to remain airborne on the return path to the a/c unit, or they may have been infected because the return airflow picked up new droplets from A1.
Air that is recirculated in airplanes passes through HEPA filters (I believe two stages) and is quite safe.
But: You don’t want to be downstream of a horizontal air current that is blowing air past an infected person who has coughed, sneezed, or talked loudly. Anywhere. Fans are just as bad as a/c units. Maybe worse. Note that no-one at tables D or E were infected, because there were no air currents from table A to those two tables (or anywhere else on that floor of the restaurant).
TY for having posted the referenced report by Lu, et. all here: Covid-19, and how to support the restaurant industry? - #191 by robert. In the report, one important finding was that there was no virus on the a/c unit. Virus-bearing droplets may have recirculated within the restaurant space, but neither they nor individual virus particles or clusters originated from the a/c unit itself. This is different in an important way from DD’s unfortunate experience with a mold-laden a/c in a test kitchen.
Also, there is a minimum number of virus particles necessary to infect. With bacteria, this is called the minimum number of “colony-forming units” (CFUs). One bacterial spore very rarely or never does it, although in principle it could, because a single spore by its lonely self is in fact a potential CFU. Similarly, while theoretically possible, it’s extremely unlikely to be infected by having inhaled fewer than some (unknown to me) minimum number of virus particles, alone, in solid clusters, or carried in droplets. Droplet/particulate density reduction through careful HVAC engineering, including filtering, are controls to limit CFU intake in an enclosed space.
As an aside with a suggestion, I used to do a great deal of business travel. If someone behind me or next to me on the airplane coughed or sneezed, even if there wasn’t anything “going around”, I would immediately hold my breath for as long as possible. That’s an easy defense to adopt, since coughing and sneezing are the primary mechanisms of covid transmission.