A tankie is generally considered someone so immersed deeply into their echo chamber of alternative facts that they can deny not merely the morality or the consequences but the actual and literal validity of occurrence of events such as the Holocaust or the Tiananmen Square Massacre (the tanks were there but killed nobody on that day, etc.). They tend to be exceedingly pro-authoritarian, even while claiming to be "communist" or "leftist", but redefining those terms to fit their particular worldview. So e.g. Ukraine invaded Russia rather than the other way around and so on. A hallmark of a tankie space is that nothing bad is ever allowed to be spoken about Russia, China, or North Korea.

I suppose claiming that Stalin had the right to kill people is a technically accurate form of tankie by the standard dictionary definition of that term, but that is fairly tame compared to what goes on across the Threadiverse, so much so that such a person would barely qualify unless they acted to prevent anyone else from holding an opposing viewpoint. Also, while some instances are legit tankies (namely Lemmygrad.ml and Lemmy.ml), troll instances such as hexbear.net only appear at first to be anything other than argumentative echo chambers, see e.g. Chapotraphouse@hexbear.net (defederated from on both of our instances, bc of the aforementioned trolling

I could link you so many posts to explain further - such as this classic Lemmy.ml tankie censorship problem demonstrating how someone can get banned from all communities (in other cases, even those they've never so much as heard of) on Lemmy.ml at once - but also there's an entire community dedicated to this topic at !meanwhileongrad@sh.itjust.works. If you are interested I highly recommend checking it out. It is perhaps the top issue across all of the Threadiverse, after complaining about Reddit of course:-).

Blocking hexbear.net and Lemmygrad.ml from my feed (when I was on an instance that had not defederated from them yet) improved my experience on the Threadiverse by 99%, and blocking lemmy.ml by a further 99%. It might be the echo chamber effect but whatever it is, the most trollish comments tend to come from those places. Note that you on Lemmy cannot do a true block of an instance - like so many other features that is something that PieFed and very few third party apps offers but Lemmy and most apps lack - but even so it might help a little.

Slrpnk.net and lemmy.dbzer0.com are widely considered actual leftist but not tankie instances, if you need a comparison.