Jpeg XL isn’t backwards compatible with existing JPEG renderers. If it was, it’d be a winner.
According to the video, and this article, JPEG XL is backwards compatible with JPEG.
But I’m not sure if that’s all that necessary. JPEG XL was designed to be a full, long term replacement to JPEG. Old JPEG’s compression is very lossy, while JPEG XL, with the same amount of computational power, speed, and size, outclasses it entirely. PNG is lossless, and thus is not comparable since the file size is so much larger.
JPEG XL, at least from what I’m seeing, does appear to be the best full replacement for JPEG (and it’s not like they can’t co-exist).