Artists can upload high quality versions to Twitter and you can download it if they do. But it seems many don't. A lot of artists seem to use Twitter to release in progress work with the final version being uploaded to Pixiv or Nico Seiga, but this isn't always true. Even if they do have accounts on other services they may end up not uploading a high quality version at all.
Knowing this, it makes sense to just upload the Twitter version (unless the artifacting is especially egregious) and just be on the lookout for a higher quality version and link the two.
The artifact rules were devised in an age when most of the art here was uploaded from other image boards who were prone to resizing and recompressing the original image. Granted Twitter does this too, but it's a different story when the artist is doing it and you can determine with some certainty if you'll ever see a high quality alternative. A poorly compressed image is better than no image sometimes.
My personal practice is to check if the artist has a history of uploading high quality versions consistently. If they do, I don't bother uploading the Twitter version.
My vote is to change the rules to discourage uploading compression artifact images but to not explicitly ban it.
