To preface: I have over 650,000 reddit karma, mostly collected from 2014-2017. Roughly 150,000 of this is from posts, the rest comments.
I completely agree that mods can push their own agenda about their subreddits; they have almost completely unfettered control of their subreddits, and the admins almost never intervene. But I don't agree that there's an argument to be made for the actual posters of content; it's simply a numbers game, as well as a good amount of "gaming" the system; e.g. posting during high-volume times, crossposting content, etc. I think the calls of shilling on Reddit are significantly more pronounced than shilling itself actually is, as I myself have posted content on multiple occasions and then been called a shill, when in reality I simply found whatever product interesting or useful or otherwise. On one occasion I was offered a free product _after_ making a post about it (I declined). I've never made a post that I've received any kind of compensation for, but I have made posts advertising non-profit events that I organized as a volunteer.
I have a group of friends that I've been on Reddit, and all of them have 100,000+ karma, and a couple with over a million, and they have somewhat shared the sentiment. I don't want to sound like I'm saying there's no astroturfing at all, because there definitely is, but there's this culture of calling anything with any kind of recognizable product in it an advertisement, when it can be explained much less conspiratorially.