The images inline in the blog are heavily compressed and look about the same. Click through to the actual demo files and the difference becomes obvious.

I can see the difference on my LCD monitor from at least six years ago. WebP really struggles with gradients. I wouldn't use lossy WebPs for photography websites. AVIF does a lot better (-25% at no perceivable quality loss), but completely messes up the brightness on my PC for some reason; I think that's a Firefox bug.

That's not to say WebP is necessarily a bad format. There are tons of images where it easily beats JPEG without quality degradation, but these images clearly show cases where it isn't.

Personally, I use lossless WebP to replace PNGs on websites, thereby maintaining lossless quality without the PNG overhead. Lossy WebPs (and JPEGs) need to be hand-checked, though.