JPEG XL is officially blessed by JPEG but otherwise irrelvant here, even though non-progressive JPEG 1 files can be losslessly recompssed into JPEG XL by its design. The main role of JPEG here was to specify explicit goals for JPEG XL proposals [1]. Interestingly enough, the JPEG 1 recompression was not a part of that call for proposals back then. JPEG XL is otherwise a completely different format with much better compression algorithms, so should be the image format for pretty much all uses once popularized.

Ultra HDR [2] is an extension to the JPEG 1 format, which depends on XMP and the CIPA Multi-Picture Format. This kind of extension was not the first, even JPEG the group itself had a similar extension called JPEG XT which was never popular! (If you don't know much about JPEG, APNG was a similarly designed extension to PNG which eventually became a part of PNG.) Naturally Ultra HDR files cannot be smaller than ordinary JPEG 1 files, so can't fulfill Samsung's needs.

[1] https://jpeg.org/downloads/jpegxl/jpegxl-cfp.pdf

[2] https://developer.android.com/media/platform/hdr-image-forma...