They probably made assumptions in their architecture that tweets can never be updated, and that they can only be created or deleted. Depending on how ingrained this assumption is, it could be quite difficult to change. Caching, replication, eventual consistency - lots of hard stuff. Until relatively recently, I believe AWS S3 didn't have read-after-write consistency for updates to existing objects - I doubt that was easy for them to do.

I've seen lots of bad hot takes from Twitter users who have no idea how large systems like this are designed. Dumb stuff like "just do a SQL update" or "use React".