Maybe Rich wasn't the source, but I don't think it can be reasonably denied that Assange was making some kind of implication there. His motives are undefined.
Since this whole discussion is obviously and thoroughly tainted by personal bias (because the information is so fuzzy that it's just about believing the entities that you trust more), I'll just lay out my cards and say that I don't believe that Russian state actors hacked the DNC (though I'm ready to admit it if real evidence is produced, which I understand is probably not possible).
I think it's a cover story because the powers-that-be don't want the average citizen to know how easy it is to compromise the security of very important institutions, whether they be political organizations or major corporations, lest that citizen decide to take up some targets of their own. I believe the same thing happened with the Sony hack that was blamed on North Korea.
It behooves IDS vendors to make people think that nation-states are regularly trying to invade their systems and it's free publicity to come out and corroborate the mainstream narrative, so they play along/feed into it and say "Yeah it was definitely the Russians even though there's no way we'd actually know that since our software wasn't installed until after-the-fact". This is especially blatant with the follow-on firms that just wanted to cash in on the available PR by saying "uh, yeah, we detected that too".
That was settled a long time ago by multiple independent, reputable security companies. It doesn't mean they were the source of the leak though.
It is fine if you want to believe their narrative. It was constructed to be believable and at this point it's just an independent judgment call on which sources are credible and which scenario seems more plausible (hacking DNC mail server by a nation-state's cyberwarfare apparatus v. internal whistleblower copying data). No hard feelings.
There is precedent for security vendors falsely claiming that nation states are responsible for some attack when it actually wasn't.
However, in this case top security vendors in direct (and sometimes aggressive competition) with each other are in total agreement, as is every public statement from the intelligence community.
Now, of course, it is in the US government's interest to paint Russia in a bad light, so claims alone aren't enough here. But look at the actual investigative trail ThreatConnect and CrowdStrike presented, and compare it to the past ~5 years of activity from what are almost definitely the same actors, and it's hard to believe they're anything other than a Russia-affiliated group.