I'd guess Assange meant that the download link he was providing was "time limited".

I don't know how you'd time limit the password itself. It isn't as if I could send you an encrypted file, with a key that will only decrypt that file for that day.

Even if Assange had made absolutely sure to destroy any copy he had of the file which used that key (which would have been a Good Idea), he could not guarantee that some router on the internet didn't store a copy of the entire download session.

Regardless of how the blame is distributed, the decision to publish such a sensitive password in such a public manner still stands out to me as the most boneheaded single action in the entire saga.