But I think in terms of federated content, you cannot be expected to do more than send information about the deletion out. If other instances don’t respect it, it’s not the originating instance’s job to police it.
It actually is.
When delegating the processing of PII to someone else (like another instance), you’re supposed to initiate a data processing agreement with them: https://gdpr.eu/what-is-data-processing-agreement/
Unless Mastodon has somehow automated this process in inter-instance communication, they are just as liable as Lemmy is.
Instance owners are responsible for the content that is mirrored on their instance through federation, so they definitely should.