I think their dates were based on the (original) April 24th flight.
I think they are morons and the date doesn't matter because they are assuming they know all strains and mutations and other factors like someone being a carrier and exhibiting no symptoms (which has happened in many diseases.)
Assuming they know ALL the contacts an infected individual had going through a city and/or airport is what is known as a fail point. Someone could've contracted the disease from the known infected individual went home talked to a neighbor who talked to someone else etc. Then he could get hit by a car and die and the doctors would not know he had the disease.
The scenarios for going undetected are endless and making a statement that a new case after a certain date is not related without qualifying all that is pure Bull Shit.