This flag reason is also only valid on third party edits and not for Twitter recompression. But even then one should find a better version first.
You're right, but to make this extremely clear: Twitter artifacts the original image a lot, that's a given. But for an image to be deleted because of artifacts, it needs to be either very bad -- on fault of the original artist (if they've really really messed up how to properly export their image), or from a third-party website or reposting site which would induce further resampling. A reupload of a Twitter sample on Twitter, for example.
Or a sample of a reupload of a Twitter sample from another Twitter sample that was reuploaded from the original tweet, for further obfuscation :^) (and this is why we emphasis links to tweets, not images!)
In any case, lot of artists make it a habit to collect their posts on Twitter first and then batch them all to upload. Most of the time it isn't worth waiting so long anyway, but that depends on the artist of course ;)
