The whole scenario is totally dependent on the probability of a potential victim being able to kill the zombie before being bitten. In some places, the zombies would win. In other places, people would be able to shoot the zombies.
They did a follow up study that considered defense. Guess what? Humanity won.
It was just a study to show who epidemics worked for a class. Nothing serious.
XKCD comic on zombies:
Outbreak
https://xkcd.com/734/