Lemmy and other services built on ActivityPub work by sending content to every server that hosts a user who has subscribed to a community or another user. Those servers could be anything from vanilla Lemmy hosted in a datacenter to an ad-hoc, informally-specified, bug-ridden, slow implementation of half of ActivityPub running on a jailbroken smart light bulb. Most of them will be online to receive a delete request and will handle it correctly most of the time, but that cannot be guaranteed.

Anything you share to the world that way is out in the world and cannot be reliably rescinded. Discussion groups implemented as email lists used to be popular, and the same was true there, but more so since there isn't a mechanism intended to edit or delete an email message after it is sent. Something similar is true of anything that functions as a public website; a great many things published to the web are available from sites like archive.org, like old forum posts.