I agree that zombie apocalypse type movies are a form of disaster movie. However, unlike WWZ they tend to focus on a group of characters who are in some way connected to each other and often the disaster itself. Be they seperated by the disaster or throw together because of it, the stories tend to be insular and concerned only with drama based on character interaction with the disaster itself (whatever it is) being little more than a catalyst. With WWZ on the other hand it's really about the war itself and it's impact on society rather than the plight of any specific person or group of people. The biggest difference is in it's sheer scope, so in a sense it's a "Disaster Movie Anthology".