Maybe Assange doesn't know who the hackers are.
He may only know know who passed them along to Wikileaks.
His credibility is now destroyed.
No more than anybody else in the media who doesn't reveal his sources (and that's assuming he knows who they are).
If he had to say exactly where every document came from (assuming he knows), nobody would bother leaking documents and his site would be out of business.
Which means Russia are the villains in all this, and perhaps Trump needs to start distancing himself from Putin?