A dev here. Not a reddit dev, but a dev. Deleting thing online doesn’t necessarily mean real deletion of the content. For instance, every post and comment is a row of a “big notebook” (a table on a database) and every row is split by columns for specific data: who’s the author, where it was posted (which community), what’s the content and, sometimes, a yes/no column called “is it deleted?”. When you delete such post, you are writing a “yes” inside that column, without actually replacing the content. It’s an oversimplified explanation of how platforms register posts, sometimes there’s a “version” table (think of it as multiple notebooks keeping track of different things simultaneously) that will keep the different versions of an edited post/comment, so they will remain intact inside such table.

Tl;dr: once on the internet, always on the internet (unfortunately). Especially if we’re dealing with a corporation that profits over user’s data. Rare cases where a thing on the internet finds real oblivion.