There are things you can accuse this picture of, but jpeg artifacting is definitely not one of them: the color blocks are completely smooth, the only thing adding colors is maybe bad scaling on the linework.
The artifacts are on the .jpeg sample, not on the original file. Also the sample is bugged; its size is 404x850 but Danbooru is displaying it at 850x1785. This makes it look much worse than it really is.