Then as a user you would be free to click to filter out comments from lemmy, and the top mod of /politics could choose to "defederate" from lemmy for that forum, and users at lemmie would be free to create /politics_tankies or whatever.

That's a matter of moderation, not the technology behind the platform.

Wait so do communities not have mods?

I'm not sure how federation does anything to prevent that from happening, though. They can still do that on your instance, from their instance.

At most, I suppose an instance could defederate from a troublesome instance that's doing this, but the more that happens, the more fragmented the Fediverse becomes, and it starts to defeat the purpose of federation in the first place.