As said, to me I'd absolutely align with stating "I believe" just not saying it like a fact. There was a way to phrase it that wasn't basically "I know for a fact it wasn't the Russians, the intelligence agencies are absolutely wrong." If there's more going on here - you're wikileaks, you can reveal that without revealing the source that the agencies are operating on pure conjecture. He can discredit them by discrediting their methods, if he has anything concrete to discredit them with. It thus far doesn't look like he does (since he'd probably leak that), rather basing it on what he thinks he knows to be true about the middle man versus what they're saying. It was that it wasn't "I believe this or that, based on what I know" it was "I'm right, the FBI/CIA/NHS is wrong- the end." While similar, it's stating two different things. If there's flaws in the investigation, he doesn't strike me as the kind of guy who would keep the flaws of the investigation secret.