GIF, or Graphics Interchange Format, is just another image format, even if it can support animation. In particular, it's an image format that works well for images with simple palettes, and was presumably chosen by this artist because it fit their art style…

…Is the comment I typed out before checking and seeing:

1. Jpeg artifacting, and
2. It's actually a webm, not a gif.

The image actually hosted on the artist's website, however, is actually a gif, with a 64-colour palette.