I'm curious about this. I'm only vaguely aware of either system, but I thought they solved rather different problems. Chef handles system configuration (like things in /etc, or "configure memcached to listen on this IP and port"). Zookeeper is more of an API to deal with distributed locking/sync issues for an application itself.
Was this interview question less of a "bingo" and more of a "one-side really misunderstood something"? Or how do Zookeeper and Chef compete?