All that's needed is the transfer rate to prove it wasn't Russians.
If the transfer rate on those DNC documents and emails that WL got a hold of where representative of a transfer from a local device to a thumbdrive, we'll say, that indicates it couldn't have been Russia. The fact that they've all but forgotten the WL documents in this case made it very apparent to anyone knowledgeable that they don't really mind a whole lot about Russian interference.
Also, PS is SPOOKy.