The problems with zombies is nothing to do with the actual AI code, but is to do with server network desync. i.e. if you run into a building and close the door behind you, if there is a delay between sync with what you did and what the server sees, on the server the door is still open, and thus the zombie comes right through.
I think this is the same with hitting through floors. If desync hits whilst you are going up stairs, the server still thinks you are downstairs, and hittable by the zombie.
Therefore, if you are on a server with no or minimal desync, you should have no or minimal issue with zombie wall hacks.