I do not know whether this has anything to do with any limitation in the WebP format, because it is more likely that this is some kind of bias in the kind of authors of Web pages who choose the WebP image format, who might also choose more aggressive compression settings, but for me WebP is a synonym for "low-quality image".

This means, that at least for the Web sites that I happen to access frequently, whenever I see PNG or JPEG images they typically have a better quality than when I see WebP images.

I do not know whether when browsing the Web I see frequently any AVIF images. They are either rare or they have a similar quality with JPEG images, so they did not stand out like WebP, where I perceived a degradation, so I noticed that the image format was different.

I do not care much about the speed of decoding Web images, because the speed of loading Web pages is determined mostly by the amount of junk JS that is included and not by the loading and decoding of the included images. So for me the quality of the included images is more important, thus WebP is something that I dislike, while JPEG is far too limited, so I would prefer JPEG XL images.

The arguments in TFA do not seem entirely honest, because the presented data confirms the low quality of WebP in comparison with JPEG XL, and because the 2 compared JPEG XL implementations differ in their performance by a factor more than 2, so it is likely that they are both immature and one could still accelerate the decoding of JPEG XL by at least 2 or 3 times in comparison with the best of them.

The only claim of TFA that seems valid, is that if AVIF is really as good as claimed, it could be sufficient for Web pages that want to show only highly-compressed images and good for replacing JPEG for such a use, while JPEG XL might be strictly required only for applications that need higher quality, i.e. for the processing and storing of photographic images.