Andrewgr2
According to the paper, the two passengers were symptomatic on the day that the flight arrived in Hong Kong which makes it extremely unlikely that they were infected on the flight. One of the cabin crew is described as developing symptoms over a week after the flight arrived making it much more likely that the transmission was in that direction. The authors allow for the (remote) possibility that the infection did not occur on the flight, but not that the cabin crew infected the passengers.