You're confusing jpg>jxl>jpg, which can be done losslessly via a special mode, and jxl > jpg, which can't (even ignoring all the extra features of jxl that jpg

You're confusing jpg>jxl>jpg, which can be done losslessly via a special mode, and jxl > jpg, which can't (even ignoring all the extra features of jxl that jpg doesn't support)