Indexes are unique to each instance. Post ID, Comment ID, Vote ID. There’s no way to correlate this information between two instances other than to do a full text match, post by post, comment by comment, vote by vote, to determine if what is being imported already exists on the new instance or is “new”.
Even if you go that route, then there’s the quandaries that follow… if you import what is effectively a “new” post to your new instance, do the comments (which aren’t yours) come along, or do you simply end up importing your post with no interaction history.
Then there is identity. You most likely have a non-local identity on your new server, as a result of federation, how does the new instance know that you are who you say you are, givimg you ownership of any of that existing content as it binds it to your, now, local identity?