I have not heard about any country denying entry or quarantining all travellers from affected areas. Isolation (at home) is recommended for people who have been in the area and present relevant symptoms. Refusing entry wholesale would be a hugely disruptive and largely self-defeating measure: it encourages people to lie, could spread the disease further by making potentially contagious victims stay longer in a public place — the airport — and through another international flight, would make (crucial) collaboration with airlines or affected countries a lot more difficult.
Extending it to all travellers from China (since you don't want to rely on people self-reported itinerary) would be pure madness (and there are already cases reported in Japan and the US… so you might as well shut down international air travel entirely). It also raises the question of returning nationals or third-country citizens, who cannot easily be refused entry and put back on a plane to the affected area.
Consequently, the question is not designed to catch people who try to defeat screening but to inform people who might not otherwise understand they are at risk and make sure they realize this information is relevant. It's more efficient than asking every passenger verbally and helps officials respond appropriately in situations where oral communication might be more difficult. It's also potentially a database you can come back to for contact tracing.