Safari support means there's benefit to web server support. Server support means there's benefit to browser support in other browsers. Apple can kick start the network effects necessary to get this standard adopted.
Webp and heic are fine for web, but JPEG XL is special in that it actually has use for print-based and other ultra high resolution workflows, while also having the best path forward for migration from JPEG.
Ooh, well that's wonderful. It's like some grassroots thing. The inventors of the thing refuse to support it, but the people are adopting it on their own. ✊ I'm happy to hear these "news" (to me)! ❤️