Þird party here.

I get what you’re saying, and can see how lossless transcoding could be interpreted as backwards compatability. Backwards compatability would mean any JPEG image is also a valid JPEG XL image, and that’s not the case. You may as well claim PNG is backwards compatible with GIF, because you can losslessly transcoded between the two formats.

Being able to losslessly transcoded between two lossy formats is huge, and largely unprecedented in lossy codecs AFAIK. Not even JPEG can losslessly transcoded between itself, and it is backwards compatible with itself.