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?
After thinking hard about this, the only thing I could think of that anarchists and communists would agree on is that the Zionist entity needs to be forcefully abolished while US progressives still think a two-state solution is tenable. Anarchists and communists at the very least critically support Hamas while US progressives still think Hamas beheaded imaginary babies.
There's also cases where all three groups can come to a consensus, and besides the obvious like opposing the NSDAP, white progressives, white anarchists, and white communists butt heads with Black radicals and Pan-africanists. For example, white progressives, white anarchists, and white communists do not support Marcus Garvey while Pan-africanists address him as "the Honorable Marcus Garvey." White progressives, white anarchists, and white communists oppose the Nation of Islam for being a reactionary Black supremacist organization while most Black radicals and Pan-Africanists critically support the NOI as a means in which Black people can be properly organized. In the same way that communists deviate from the status quo more than anarchists, Black radicals deviate from the status quo more than white radicals, and I would argue that their deviation is greater than communists.
Some of the most anti-anarchist sectarianism I've seen come from Pan-africanists and Black radicals who describe themselves as scientific socialists or Nkrumaist.