Lemmy and other federated systems being spread out across individual instances does make things more difficult. Normally you could just do “site:reddit.com” and automatically filter all results to be from Reddit. But you can’t do that because your result could be on any one of hundreds of instances, many of which do not have “lemmy” in their title.
It appear reasonably large instances like lemmy.one have been indexed, i got results using site:lemmy.one. and for those larger instances, they should still be able to index federated content.