“Day (weeks) late, dollar short” applies. Cat is out of the bag now...further testing will just help track the community spread, not quarantine/isolation planning....unless you do the Chinese Communist Party thing of mandatory quarantine for all citizens in a city/area with draconian consequences for non-compliance.
Basically, yeah.
I guess it can help you figure out which cities to quarantine first—but you have to be ready to go there or there is no point.