Can someone please explain the nuance of federation that allows this? - eviltoast

So here I am sitting on lemmy.ml which is defederated from threads (and which I chose primarily for that reason) and I see a comment from the user above.

That comment is on a community at lemmy.world, and the user is apparently registered with their @threads username through @sh.itjust.works.

IIRC .world does NOT respect the fedipact, and I guess this means sh.itjust.works must not either, but I’m super confused by the username.

So I guess I still need to just block all the threads users I see, defederation or not?