City population is 11.9 that would mean that 1/2 the population left...I don’t think so, it would take mass coordination.
The only way this would be feasible is if he is referring to the normal mass migration that happens in January prior to the new years holiday when factory workers in the cities head back to their home towns for the holidays.
I have experience with this as many of the factories I’ve dealt with have issues getting back up and running in late Feb/March due to workers not returning from their homes to retake their positions and instead looking for greener pastures.
However, If he is referring to that then the timing is off a bit from when the virus was first reported...based on my experience.
It’s probably that that many people came and went from the city during normal work and travel in the time period before the emergence of symptoms among people, otherwise it would have been one of those “in other news, half the population of a major city bugged out recently” story...