They captured someone who knew the codes and who THOUGHT they'd want them from him and thus abducted him. So he told them, because he was desperate.

HOWEVER, they obviously abducted people without any plan but 'studying' their bodies. This whole tape is to show how the aliens are completely inhuman and uninterested in human history and life and how you can give them NOTHING that would somehow appease them. They didn't care for the codes, hell I bet they didn't even understand what he said.