It's better than AVIF at low bitrates. I did a comparison of the latest builds 3 months ago, and was amazed that JPEG XL has more detail than AVIF at 0.05-0.1 bpp (~120KB 1080p). It's a bit subtle, but I could see it. Once browsers ship full support (at least Firefox + Chrome), I'll be on board.
bpp = bits per pixel, not bytes. At 1080p, "0.05-0.1 bpp" is 13-26 KB. Much smaller than what you were testing. Might want to recheck your sizes, if you are actually looking at 0.5 bpp it would not surprise me at all that you found JPEG XL superior.
At very small sizes like 0.1 bpp the results are debatable, but I think AVIF is a pretty clear winner if you aren't too bothered by blurring. Samples at 0.15 bpp:
https://afontenot.github.io/image-formats-comparison/#air-fo...
https://afontenot.github.io/image-formats-comparison/#citroe...
https://afontenot.github.io/image-formats-comparison/#festa-...
Document-Policy explainer: https://github.com/wicg/document-policy/blob/main/document-p...
I tried it out on my own site, and through trial-and-error I found that Chromium does in fact treat the "max-bpp" Document-Policy directives as bytes-per-pixel.
I could be wrong; my memory has faded. Please correct me if this is the case.