How does Lemmy and federated communities deal with conservative terrorism?

There's more than a few that have just been mass defederated by most instances

Same as most places. Moderation, ban hammer, and for Lemmy specific solution, defederation with a problem instance.

Hexbear and lemmy.ml both had their share of brigading and trolling to deal with. It will likely become worse as it grows more popular.

The modlog is a useful tool, though it doesn't capture everything

the reality is, there will end up a pocket group with this and other types of content, over time it will be effectively a shadow network. the blocking game will always be a thing.

making meme’s obviously

Every instance will need to take moderation seriously if they want to stay federated.