The problem with that World War Z excerpt is pretty straightforward:
Slow zombies can't climb. So they have absolutely no way around or over any obstacle the height of a man or taller.
Slow zombies also don't have the brain power to detect traps.
You set up tall barriers, you lure the zombies up against those barriers by waving at them or what have you, and you kill them. All day. Or once they're stacked up against your barrier a kilometer deep, you helicopter out and napalm the site.
An enemy that can't hide, can't retreat, can't stop himself from advancing, and can't overcome simple obstacles isn't really an enemy. He's an annoyance.
Brooks made that battle work by having the military be too dumb to set up a simple stone wall 8 feet high in front of the advancing zombies. Or if there was no time for stone, city busses lined up nose-to-tail.