I suspect they mean that inbound flights from international airports may still take off (probably in part because the FAA can't _really_ stop them, in part because the volumes are much lower and thus more manageable, and in part because one might hope that by the time it takes to get to the US from many international destination, the incident will be resolved).
And I assume it becomes case by case at some point because they eventually start running out of space on the ground at airports.
Very unlikely that any international flights bound for the USA are taking off. Probably many are turning around or diverting if they are not more than halfway there.