Machetes work just as well, if that is all you have (see Rwanda). I imagine rocks and sticks could be used if your civilization has not got to the universal machete ownership level.
It's talking about genocide. If internal strife gets to the point of extreme hatred, then you don't need guns to kill each other. Not saying I agree that would happen in the U.S., but that's what they're talking about. Not Vegas.
To reiterate: See Rwanda. https://en.wikipedia.org/wiki/Rwandan_genocide