> You honestly can't fathom a situation where Assange would know for sure Russia wasn't involved?

I can certainly imagine situation s where Assange would know this, but Assange hasn't claimed that Russia wasn't involved, by his own claims even when a pardon that would erase the risk he has been fleeing desperately from for years was dangled in front of him as an incentive, so I think it's pretty clear that either that situation does not exist or he has an undisclosed reason to preserve the image that Russia may have been involved. The former seems to be the simplest explanation.

> knows they don't work for Russia (because the person already has high-level clearances/was rigorously vetted/etc.)

World powers have had highly placed agents in foreign governments and political parties, etc., with high-level clearances that require rigorous vetting numerous times in the past. So, no, the person being in such a position does not establish that they are not a Russian agent or that Russia was not involved in their actions.