(Sorry about editing the rug from under you - I write these comments by adding to them bit by bit, but it somehow only 'works' if I do it live.)
You're raising an important point which I think about a lot. It's very clear to me that most users who get into intense arguments about divisive topics are sincere. The anger and hurt feelings that energize these arguments and get re-energized by them are too unmistakeable to just be cynically planted. But it may be that these are second-order effects of propaganda systems. If you convince enough people and get them feeling angry and hurt enough, they'll do your job for you, and much better, by being intensely sincere. You can't say that these commenters are manipulators, because they're not. "You're a shill!!" will meet with justified indignation, and "you've been brainwashed!!" is no good either. The whole distinction between "manipulated" vs. "authentic" doesn't address what really goes on in these discussions. You can't use it to make anything better. That's not to say that we should do nothing when we find some real indication of actual manipulation—the opposite is the case. But it doesn't make that much difference when the 'organic' dynamics point in the same direction. Certainly it doesn't solve the problem.
Another extremely powerful factor that does the propagandist's work for them in an entirely innocent way is tribalism. Is it manipulation if I feed your beliefs about your identity and (especially) the opposing identity in a way that inflames you with indignation against the enemy? That "food" is so eagerly gobbled by the consumer that at some point you have to ask who's influencing who. The tribal psyche, which we not only all have but are almost totally dominated by, is enough to generate the situation all on its own. Media stories will feed it because that's what people want. No one can get enough of hearing how good their group is and how bad the opposing one is. This force is so powerful that it's not even possible to discuss it, except in the abstract.
In my experience, if you take that seriously then you come to feel that such external manipulators as are lurking out there are only effective because they activate the real manipulators, which are inside ourselves. "We have met the enemy and it is us," etc. This perspective is much better because it is empowering: we can do something about it, we can work on it, we can change. The other perspective (external agents are manipulating!) just leads to a wilderness of mirrors, even if external agents doubtless are manipulating. There's nothing we can do about that, except in the important (but relatively rare) case where evidence appears; in the absence of evidence, all we're doing is projecting our shadows onto each other.
The reason I think the sufficiently-smart-manipulator argument is a good one is that besides being the only response that can actually work, it points us directly at what we need to be doing anyway, which is addressing these more powerful factors in ourselves.
Ok, I'm done editing for the moment :)