But I would expect anarchists to be able to see that the modern communist movement by and large has recognized the faults of its ancestors

It absolutely has not. I've seen no indication of this. The maoist group in my city has written articles about how Stalin didn't do anything wrong, has infiltrated groups in an entryist manner, has spread anti-anarchist propaganda to try to make people choose between them and the anarchist groups, and, in doing so, has turned people who have worked together forever against each other. They even regularly "joke" to anarchists who still work with them that "after the revolution" they are going up against the wall, or into the gulag. But it is one of those jokes you can tell isn't really a joke...more like a "it is funny cuz it's true" sort of joke.

We're not about to have a communist revolution any time soon, so in my opinion it really takes a certain type of privilege to say "we're not going to work with this other group because when the socialist revolution comes someday, they might be mean to us".

If we're not building a movement that is explicitly anti-authoritarian, then we're just wasting our time, and I'm out. Actually, that's not quite right -- I'm not "out", I'm just going to be elsewhere trying to build anti-authoritarian resistance to capitalism away from people who are okay with authoritarianism under a red flag.

What good does it do to say that you won't work with anyone else that doesn't align exactly with your politics?

I'm not asking anyone to exactly align with me. I'm just asking them not to be okay with authoritarianism of any kind. That's it. They can be marxist, mutualist, christian, syndicalist, ancom, postleft, libsoc, etc etc -- as long as they are against capitalism, hierarchy, racism, and other forms of authority, I'm cool.

And, honestly, if you want a mass movement, then asking people to be okay with tyrant loving authoritarians, just because they wave red flags is a good way to turn people off and make them not want to have anything to do with you. It would certainly have that effect on the working class stiff typing this message right now for instance -- and I'm by no means alone in that regard. And that's not just a unique trait of anarchists -- you'll find a lot of non-ideological people in the general population feel the same way. I mean, why should they stick their neck out against capitalism and systems of oppression over them if all it means is that a new class of exploiters under a red flag will be exploiting them instead? Certainly seems like common sense would see that as a fool's errand.

And, yeah, a few years ago, my experience in my local groups was similar to what you describe. Then the Maoists fucked that shit up with their divisive entryist tactics and stalin loving rhetoric. I still judge individuals as individuals, and I still will work with the authoritarians in a very cautious and provisional basis -- but if you want left unity, you need to stop trying to convince anarchists to let authoritarians be authoritarian, and go convince the authoritarians to stop trying to control things now and to not have centralizing and ruling over others as a goal that they are trying to build.