Edit for clarity: I'm not asking why the Tankie/Anarchist grudge exist. I'm curious about what information sources - mentors, friends, books, TV, cultural osmosis, conveys that information to people. Where do individuals encounter this information and how does it become important to them. It's an anthropology question about a contemporary culture rather than a question about the history of leftism.
I've been thinking about this a bit lately. Newly minted Anarchists have to learn to hate Lenin and Stalin and whoever else they have a grudge against. They have to encounter some materials or teacher who teaches them "Yeah these guys, you have to hate these guys and it has to be super-personal like they kicked your dog. You have to be extremely angry about it and treat anyone who doesn't disavow them as though they're literally going to kill you."
Like there's some process of enculturation there, of being brought in to the culture of anarchism, and there's a process where anarchists learn this thing that all (most?) anarchists know and agree on.
Idk, just anthropology brain anthropologying. Cause like if someone or something didn't teach you this why would you care so much?
A big part of it is that it's more aggrandizing and exhilarating to see yourself or your entity or alignment as "against the whole world", instead of "positioned in calculated opposition to certain things and somewhat aligned with the rises and falls of many historical formations", many of which take a whole lot of convincing and nuance because of how many pitfalls they fell for.
Someone really diving into an anarchist identity (not a meaningless liberal one) will either draw on histories of anarchists being suppressed or otherwise at odds with the USSR, or on much more recent queer and punk and insurrectionary authors who are critical of all party formations that seek to contend with governing atop entire countries. The latter will typically identify the CPSU as the most successful (fwiw) of these, and readily associate it with many modern-day communist parties, which they do have more of a direct experience of.