I guess you have a misunderstanding here, it's a lossless compression of an already lossily compressed file (which had used suboptimal coding methods). You will

I guess you have a misunderstanding here, it's a lossless compression of an already lossily compressed file (which had used suboptimal coding methods). You will get a bit-perfect original JPEG file when you decompress a recompressed JPEG XL file. No additional compression artifact there.