Also not a lawyer, but right to be forgotten applies to search engines to remove articles from the search index. Originally applied to news articles some guy in Spain didn’t want showing up when you google’d his name. The law doesn’t require the publisher to remove the content from their website, but instead requires search engines to remove the links in results.
So if someone’s comment was mirrored to Lemmy AND that comment was indexed by a search engine linking back to a Lemmy instance, then you still have the right to request Google or Bing or whatever to remove those links from search results via the same process.