As always with cases like this we need to review the actual situation, not just distil it down to a hyper-simplistic minimal summary and then base a conclusion on that.

It's not just a case of believing what the CIA say, they and other independent cyber security teams who have also investigated this, have also presented evidence to back up their investigations to show why they think this. There are also other cases of known proven Russian hacking. Assange has provided nothing. So let's not pretend this is a symmetric case of word versus word.