Just because they're dumb doesn't mean they're not capable of doing damage. It isn't so much about the intelligence, but the intent. A killer instinct is still a killer instinct, and for zombies - or walkers, in this case - all they know is the need to feed. They will do whatever it takes. Sometimes not having intelligence as a distraction can be helpful.

Not that it's right, but it's true.

"Oh. So kind of like zombies then." He nods. "They don't really have those where I'm from."