Barium meals
The Mueller report only gives the official narrative, which was a Brennan and Crowdstrike parallel construction. Guccifer 2 gave Assange a document that he already had from the Podesta emails anyway, for instance. Also some DNC documents. It allowed them to claim Russia was the source.
It's why they waited for the Mueller report to be filed before bringing Assange in. Now he can prove Brennan lied.