I don't disagree with your points or perspective, personally I didn't bother investigating much of the viewpoints in the communities on lemmygrad to the degree that you did. Mostly I knew it was more based around communism and beyond that I wasn't necessarily interested in specifically what the communities were about.
My mention of echo chamber was not to criticize you for wanting to exclude, which is why I mentioned I had already blocked their communities. It was more of a general statement about how I blocked them, but overall my intention isn't to just block everything I disagree with and put myself in an echo chamber. But what you described is why I was comfortable blocking it without doing the investigation you did. Because I saw on join lemmy that the instance was specifically labeled as "a collection of Marxist communities" and I disagree with the notion that people should collect together or identify collectively in that way because it often leads to the type of activity or thinking you described you encountered. People often begin to allow their identity to tell them what to think about something or view about something and in an echo chamber of other people doing that in my opinion it often ends up becoming like the flanderized characters on shows that run too long. It usually is something like 'I'm an artist, so obviously I take the side of whatever I think most aligns with what I think being an artist is because it reinforces to me that I'm an artist' and people don't even consciously realize that they're doing it. That's a lot how flanderization of characters works. So without diving into lemmygrad, I could reasonably guess that the type of content or viewpoints you described would exist there because I understand how humans who gather together around identities end up behaving.
As far as Nazi's holding the door or whatever the case is, if someone who identifies as a Nazi is capable of respectful and responsible behavior in public and follows all the rules of being out in public, then why should I restrict them from going out in public? Mind you I know you didn't say you wanted to restrict them, I'm not claiming that was your argument, but if we try to compare it to IRL then generally speaking either you have to inconvenience yourself to avoid a Nazi in public, or you have to restrict a Nazi from going out in public to avoid them, there's no realistic way to do it otherwise for IRL. Now on the internet, there's an expectation that there is a middle ground because it's not bound by the same limitations of real life and I think that's what you're saying that you're wanting. Again, I also blocked their communities so I don't think it's crazy to want that, but the development of the platform is starting from scratch and as I mentioned in my other comment, there is an open issue for it on github but it could take time before it gets implemented.
I don't know that there's really much more to say on it other than dessalines seemed to OK the idea, and there's sort of a "plan" for it to be implemented in that there's an open issue on github, and all anyone can do is either be a developer and code the solution themselves, or wait patiently for someone else to do it. Beyond that, in the meantime the only other thing you can do is sign up on an instance that is not federated with lemmygrad. Sopuli.xyz and feddit.de were some others mentioned that aren't federated with lemmygrad.