Looks like it was just for the convenience of specifying each color with 3 hex digits instead of 6. I doubt it was a requirement, just a nice to have. And the difference between 12 and 24 bits is negligible in this application.

I also often limit myself to 3 hex digits. The colors look very similar (except the really light ones), and it's more convenient to work with 3 hex digits than with 6 hex digits.

Seems to have been for CSS and SVG bandwidth golf. The article mentions it being for this site: https://grid.iamkate.com/

According to the bottom of it, it can get quite a few views each day, so bandwidth savings may visibly effect hosting costs. The main CSS file is only 5kb compressed/15kb uncompressed, so it seems to have been part of a larger effort in minimalism.