Sadly, Google Search doesn't seem to index Lemmy instances the way it does Reddit.
Sure, you can add "site: lemmy.world" to your search strings, but that mostly returns links to community or user feeds that mention your search terms, not links to the actual posts. And it doesn't group the results together for convenience, the way it does for Reddit.
Hopefully, this will change as Lemmy becomes more popular. We could also outright ask Google to start treating Lemmy this way.