Much like WEBP, JPEG-XL is two different codecs in one. There's VARDCT mode, and Modular Mode. Modular Mode is normally used for lossless, but it can also be used for lossy as well. When you use Lossy Modular Mode, you get different kinds of compression artifacts than what you'd see in a JPEG file.

For Lossless: JPEG-XL wins, then WEBP, then PNG. (Sometimes lossless WEBP wins)

For Lossy: AVIF wins, then JPEG-XL, then WEBP, then JPEG.

Lossy Modular isn't efficient enough to compete with even JPEG.

Also, look at the graphs – Iris-WebP beats JPEG XL.