I don’t think he said that at all. He said that WikiLeaks was researching this and funding a reward for the killer “just in case” people “thought” he was a WikiLeaks contributor — that WikiLeaks would defend anyone who was unjustly targeted (whether they really contributed to WikiLeaks, or not)as a way to defend all those who really worked for WikiLeaks. Convoluted, I know, but did didn’t really expose Seth Rich as a contributor — left it up to interpretation. It was a big nothing, IMHO.
Is it not obvious that if Seth Rich were not the source, Assange would simply say so?
Rule of thumb: refusal to confirm or deny generally is an indirect affirmation. Because if the answer were no, there would be no loss or risk in denying it.