1. He can't possibly know if Russia is the true source.
2. Seth Rich is a classic KGB style conspiracy theory with literally not one shred of evidence, at all.
So he's doing two very odd things here that he's never done. He's saying Russia is NOT the source AND Seth Rich IS the source.
It's typical for someone with good intentions to find themselves owned by a spy agency. Assange is most likely in so deep he can't fix it.
It's a fact he's taken money from Russia and the theory of him being compromised by them goes back years before the election. He's the one that arranged for Snowden to go to Russia.
So he's compromised and a tool now. It doesn't matter if he was once free or not at this point.