First, check this. One thing is for sure, those are not artifacts (too localised). The art contains plenty of jpeg artifacts, but those glaring squares are not them.
Second, if this were a scan, even that wouldn't be passable. Here is my 1 minute edit into a 16:9 4K UHD (3840x2150) wallpaper. This CAN be passable and edits are not discouraged, so you can upload an optimized version. All I did was open it in Photoshop, downscale it, adjust a bit of brightness and contrast and throw in a sharpen.

The artist is very good at what they do, so I am not sure why this image is presented in this poor quality. From their comment on the art I am left to understand that they made it in a rush?
edit: even if they made it in a rush, this does not explain the final output file being like this.

TLDR: Nah, it's going. Too blurry as Em said.