Hello friend, Lemmy isn't quite big enough that you need to post these twice, as it is, it will get seen even if you post to an entirely dead community. Multiple times just takes up space
I don't know if they posted to a community on an instance not federating with Lemmygrad but I don't see another post from their profile. But no, it's fine to post to different communities. That's also why cross-posting exists
Different instances are federated with different groups of other instances. By posting to multiple instances it makes it more likely that people would see the post that they would have otherwise not seen.