Comment by JyrkiAlakuijala

Ideally one would use human ratings.

The author of the blog post did exactly that in a previous blog post:

https://cloudinary.com/labs/cid22/plots

Human ratings are expensive and clumsy so people often use computed aka objective metrics, too.

The best OSS metrics today are butteraugli, dssim and simulacra. The author is using one of them. None of the codecs was optimized for that metrics except jpegli partially.