It's listed as an example to illustrate a major catastrophe that includes the most common effects of a major event that could severely damage US economically and militarly.

Take a better look, and you'll see how a supposed zombie plague has everything that could go wrong happen at the same time (giant angry mobs, death of millions due to a biological or chemical agent, said agent being a foreign weapon, widespread collapse of energy and communication infrastructures, miliary intervening with martial law to try and contain the collapse of social cohesion in the most densely populated areas, etc,etc.)

FEMA has their own think tanks and planners who study and analyse these scenarios thoroughly every day, in order to plan out the protocols that should be engaged in case any major natural or human disaster hitting the US. Of course, the plans themselves are highly classified info, as to prevent our enemies to finding workarounds or weak spots in them.

The disaster could actually be something like a flood, or a hacker attack on the power grid, a nuke being detonated in american soil, a land invasion via Canada or Mexico, a pandemic (not the scam that is covid, think airborne Ebola, or a second Black Death scenario).

Zombies, in a nutshell, only represent a fictional scenario that serves only as an illustration of the kind of planning the government has in case of a disaster, and to show that a response has already been thought of, and will be engaged on a moment's notice.