I think it's absolutely true that social media and other online platforms are prone to an overly heated, tribalistic discourse that made, and still makes, the extent of 2016 primary divisions appear and feel larger than they are in real life. Proving that Russia was responsible for that in any meaningful, quantifiable way strikes me as a very hard thing to do, though. People were invested enough in Clinton and Sanders for this to be a heated primary even without any Russian involvement; certainly, the 2008 primary didn't need Russia to be what it was.
Moving past 2016 to the present, I also have to disagree with any framing that implicitly or explicitly places equal or greater responsibility for the perpetuation of post-2016 divisions on the left, as I think yours does. Obviously, there is a vocal portion of the Sanders camp that's unproductively fixated on relitigating 2016; I don't think anyone can dispute that. But it's plainly clear to me from the tenor of the intra-party discourse around Sanders, compared to the ~80/10 fav/unfav that he consistently holds in intra-party polling, that the anti-Bernie wing of the party (in case this isn't obvious, I mean those who see him as some sort of intrinsically malign figure who cost Hillary the election, not people who've merely criticized him here and there or just didn't support him in the primary) is grossly overrepresented among the Democratic political classes, meaning center-left pundits, think tankers, consultants, etc. I'm going to keep putting the majority of the blame on the side that has the much greater institutional power.