I am sure there are examples where JPEG XL outperforms AVIF. But looking through all those images in the test sample I get the impression that AVIF on average in the lowest quality setting is significantly better than JPEG XL.

I agree that the example you presented indeed has the mentioned shadow issue in AVIF.

If tiny quality was 1.0 bpp, low quality 1.5 bpp, and medium quality 2.5 bpp and high quality 4.0 bpp, JPEG XL would win in all categories.

In reality cameras produce about 3.5-4.5 bpp, and the internet uses an average of 2 bpp for photographs.

Realistically AVIF would mostly be used to reduce file size relative to JPEG, not increase quality.

It does but the middle bit looks far better, AV1/tiny is comparable to JXL/large when only looking at middle bit, weirdly enough, everything around is of course worse

https://imgur.com/a/PeHlyE4