Fediverse

The person who cross-posted it was ~~probably~~ definitely from your local instance.

You only ever interact with your local instance's copy of any community, even remote ones. If the community is to a remote instance that is either offline or since de-federated, there's nothing that prohibits you from interacting with it*. Because lemm.ee is no longer there to federate out the post/comments to any of the community's subscribers, only people local to your instance will see it.

*Admins can remove the community and, prior to it going offline, mods can lock it. But if an instance just disappears, you can still locally interact with any of its communities on your instance; the content just won't federate outside your instance.

Looking back through your history, that's a post by a user local to your instance. You can see it because you're on the same instance.

If I understand how federation works correctly, posts don't go directly to the instance a community is on when they are made. They are created locally on your own instance, and then federate out if/when they can. Since you're both on the same instance, you can see the post and interact with it, but the post and your comments are (presumably) stuck in a queue trying to federate to the now defunct instance. Since lemm.ee is gone, it can't federate out, so other people don't see the post/comment on their instance.

I think that's what's going on.

It's unfortunate that there's no way to see that a community is on a dead instance - lemm.ee communities will still show up in searches, show that they have subscribers, and look exactly like any other community, including letting you create new posts.