This is b/c you are on lemmy.world, which defederated from both lemmygrad.ml and hexbear.net. So you only have the likes of lemmy.ml and midwest.social to contend with, which are nowhere near as extreme as those others.
Also, by the time of the Rexodus a lot of that drama had already been hashed out, hence posts like the OP are rare these days. But it is preserved if you want to seek it out.
A lot of what OP is talking about is the "unexpectedness" of making a comment that might seem reasonable to them (like "the Tiananmen Square massacre actually did happen tho"), and then shockingly they get banned even from communities that they had never even so much as heard of, never mind visited. The admins of that instance are VERY eager to whip out a VERY heavy ban hammer upon their slightest whim. Which is... fine, it's theirs to do with as they please, but some newer people (like OP) are shocked at how anti-democratic that seems, and wish that they had been told. Especially since if you read the sidebar of places such as lemmy.world, you would not expect that behavior - all the more so from an admin instance. Hence they tell others. And then new people join, and the tanky vs. anti-tanky cycle repeats:-D.
The hexbear community predated Lemmy iirc, so they have existed in an isolated bubble for a very long time. Imagine edgy teenagers with an axe to grind and no particular reason to avoid grinding it on anything that they set their sights onto.
Despite running off so much of their own community members, including some actual developers, they seem happy with the way they are. Sadly, they are also happy to spread out from their instance and fuck up everyone else's day as well - rulez be damned. Notably, they continually keep floating the idea of defederating themselves from the rest of the Fediverse - I mean ofc those instances that have not already cut them off - so if that gives you an idea of what is going on (they are aware of their toxicity, they simply choose to not care).
That particular community might be fine though. Or not. Either way it's probably more tolerable than many hexbear users that you will see in general across the Fediverse. You'll see for yourself.