"tankies" in the way they're using it is basically anyone they don't like to their left (it was originally an intra-left sectarian insult, but the liberals using it don't care about the history). But what usually sets them off is the assertion that US hegemony is an incredibly negative force worldwide, and that it's so bad that consequently multi-polarity (ie increased influence by US enemies and non-aligned states, russia and china being especially controversial) would be a very good thing, even if the individual countries gaining power are imperfect, or outright "bad", because it weakens US hegemony (which is a primary factor holding back socialism worldwide). In a multipolar world, non-aligned countries could do much better for themselves and their people rather than just be heavily exploited by the US and its allies, and no one power would be able to dictate terms unilaterally.

In terms of "authoritarianism", sure, there are aspects of implementing communism that require ruthlessness, I think even anarchists would agree. The capitalists (people that own companies and hold power, not just people that "believe in capitalism") will not let go of power without a fight. But our goal is a society that would actually encourage human flourishing and guarantee a good life to everyone, not one of wage-servitude to another just because they own the land, or the company, or tools. I would not really classify that as authoritarian in the way that word is commonly used, at least not when considered from a human perspective. There's more to human freedom than freedom to accumulate and spend money unrestricted by the state or even the needs of other humans.

The shitposts and edgy humor are a mixture of things, some are venting/exaggerating, some are mocking liberals, fascists, etc., and some are just surreal/weird humor, but what they generally are not is ironic. People aren't ironically in favor of the maoist uprising against the landlords, nor trans rights, nor guillotines, nor the soviet union, nor whatever else you're seeing here. That doesn't mean we support those things we meme about uncritically (see also: Chris Dorner memes), but the general disposition here is earnest, believe it or not, especially the part about advocating communism.

As to tankies being just like the far right... I don't know how people take that seriously unless their understanding of politics is so simple as "good thing/bad thing" and they hear "china bad" "cuba bad" "commies bad" so much on the news that they just believe it. They come to us saying things like " you're communists? but commies are bigoted and don't respect human rights" like it's just a fact because they heard it a bunch of times, and it's like, "well do you see that here? do you see us being racist or queerphobic?" We still criticise countries that we like on their policies when we disagree with them, and we do so with the understanding that unlike capitalist nations, which use minority rights as a bludgeon while not addressing the roots of inequality, communist countries have a lot more capacity to be genuinely responsive to their citizenry, and to nurture attitudes that attack the roots of bigotry too. The new family code in cuba is a huge transformation for a country that liberals love to slander as homophobic, for example, and it's supported by their people.

As far as replying with emotes and memes... sure, we do that. It's (usually) derision of the ill-considered liberal (or worse) ideas being espoused. But I don't think it precludes having a deeper discussion if a couple stray emotes slip into the replies. Hopefully fixing the emoji sizing issue on federated instances (which our devs are contributing a patch for upstream) will reduce the perceived severity of this infraction upon the eyes of other instances. I'd say its also partly just a culture shock. We've been over here replying in emojis that we mostly all know the context and history of for 3 years now, whereas there's no culture of replying with inline images on reddit that I recall.

But as for why some posts/comments get such a derisive response and some don't: there is a line between someone who is asking a fair question in good faith, and someone who is hilariously ill informed and/or fishing for a gotcha, and generally liberals don't come to conversations about, for example, communism or a certain ongoing war with an open mind at all because "putler and Xinnie the pooh are straight up evil how dare you even compare that to the merely flawed USA". Our aim isn't perfect, but the IMO the goal generally is to respond to the former with sources, criticism, and argumentation, and the latter with outright derision because they don't deserve our good faith if they can't offer the same.