I think his question was more directed to other cards that target defense cards. Cards that say "Get a defense card from your discard pile", would still be able to grab Zoo Keeper.
However, you're correct, the defense power cannot be triggered if Zoo Keeper is in your hand.
Yes, your situation was what I really wanted to bring into question. Thank you for providing further clarification that I should have in the original post.
Thank you all for your responses!