I've noticed the same issue with WebP and have gone back to JPG/PNG for most things (jpg for photos, png for UI-type images) I think the real problem is, like many of the commenters here, most people can't tell the difference because desktop monitors have been stuck in a deadzone of zero innovation for the last 10 years. I'm sure half the folks here are viewing his example images on a 2012-era HD 1920x1080 LCD, which…
I have an extremely hard time perceiving any difference on a 27" 4K monitor. I am not even sure I really see them. The examples are just bad. If you want to show something, screenshot and enlarge it to show the artifacts.
One might argue that if you need to enlarge it to see the artifacts, then the artifacts aren't perceptible enough and the codec is already good enough for the use case.