Wrote this as a comment, but was too long. Feel free to disagree.
I think that lemmygrad should be defederated, but i think that lemmy.ml should not be defederated, not for now anyway. The vast majority of people on lemmy.ml are not tankies, and politics or tankieism is not a major topic of discussion on lemmy.ml.
Idk if this is true, but I heard that they suppress anti-ccp views, but as long as they don't defederate from other instances, you can always just post them here or on whatever other instance and they'll still be fully visible from lemmy.ml (i'm pretty sure this is true but not 100%, still pretty new to all this fediverse stuff). If they did however start defederating from every instance that allowed truth about "left" auth governments, then yeah ok you can defederate. But that is not what lemmy.ml is doing, at least not right now.
There's different rules based on instance, this will probably be a sticking point and has the potential to derail lemmy entirely if every instance is only federated with the "correct" instances. Lets say lemmy has 100,000 users, its not that much yet but for example. If there's 10 different "networks" that only talk to each other from that same network that has the same rules (obviously, bigotry/"don't be an asshole" rules need to be enforced for every instance), that site is doomed to failure vs if there's 1 network with everyone talking to each other and generally agreed upon default communities for each topic, or even the idea of "multireddits" in whatever form.
I'm not saying federate with every instance, I'm just saying it should be a HIGH bar, not a low bar like a differing signup policy. Being focused on porn makes sense or even if an instance was 50% porn, that'd make sense. Or obviously if there's bigotry, extremism or violence coming from an instance. Which lemmygrad.ml passes, but lemmy.ml doesn't.
If you defederate based on small things, then there'll be 10 gaming communities, 10 NFL communities, 10 "ask reddit" communities. Which is not sustainable obviously. This was one advantage of reddit, it was a "hub" that had 1 (maximum 2 if a split, there was never 3 that were totally equivalent for any topic) forum for literally every topic in the world. A single for profit company controlling pretty much every equivalent to a 2000s forum on the web was very convenient, but was always going to end badly.
Beehaw.org just defererated lemmy.world and shitjustworks, because of the open signups policy (as opposed to the waist high fence of a few paragraphs explaining why you're not an asshole)
That's a major mistake imo and i don't think I'll be using that account as my "main" anymore. Not like I dislike the instance or anyone from it, but a lot of lemmy is now invisible entirely from there.
TL;DR defederating should be used only when you are fundamentally opposed to the core of an instance. Otherwise the lemmy universe will fracture and fail