Clearly this is not factual but looks more and more to be just made up. That won't stop posters like Space Ghost repeating the lie over and over and over for the next 3 months. I wouldn't be surprised if Trump starts claiming this as the truth. It's pretty much their MO - make stuff up and repeat it until people buy into it.
Hillary pretty much admitted that she said this. She could have said "NO, I never said this" but... She said "I don't recall joking about..." Her answer basically said, if/when the evidence comes out, then the comment was a joke.
I still haven't figured out what was wrong with suggesting we drone Assange, joking or in all seriousness. There's obvious faults to such a plan, but he has essentially set himself in opposition to our national interests. It wouldn't be beyond the pale to consider him an enemy and to treat him as one. I do think droning would be excessive because he lives in a friendly country with effective law enforcement through which we can pursue legal action against him. But, if he was running loose in enemy territory leaking state secrets left and right, I wouldn't even blink at the idea of targeting him. Snowden's case is a bit more complicated, but Assange isn't even a citizen, isn't concerned with the best interests of the American people, and it seems to me actually interested in causing as much damage as possible to the national interest. I see no problem at all with Clinton taking a hostile stance on the subject.