JPEG XL has many compression modes inside, and the lossless JPEG roundtrip works only for one of them that was specifically designed to hold old JPEG's data. You can't take an arbitrary JXL file and turn it into a JPEG without lossy recompression.
The old-JPEG-preserving compression mode also isn't the best that JXL can do. It's better than the old JPEG, but not as good as JXL using all the new bells and whistles.
The old-JPEG-preserving compression mode also isn't the best that JXL can do. It's better than the old JPEG, but not as good as JXL using all the new bells and whistles.