I would think that general pathway patterns laid out should lead either a single zombie or a group of zombies around obstacles to you or whatever their target is. So it's more of a recognition of landscape thing, and like I said, it's not like it's AI for every individual zombie. Sure, some smarter enemy types can have different patterns and whatnot. But AI isn't a situation where every single sentient being (if zombies could even be called that) needs an individual "brain".
Now that I think about it, if everything in the game can be used as a weapon, there might be more physics processing than zombie AI. Now that might be an issue.