The problem is that the lemmy.ml admins decided to bulk delete these posts from their database directly, which sadly doesn't federate out to other instances, and thus the only way to solve this is for other instances to also delete them directly in their database.
Bulk mod actions are unfortunately a missing feature in lemmy at the moment. Consider donating to the lemmy devs and/or let them know that bulk actions and other modding tools are important features that users want.