It's not really a "mode", it's a completely different codec that appears to have been invented by a single random guy at Google and they just released it one day. Much like the rest of WebP.
I'm aware of that, search for my other reply to this thread. Also that single random guy [1] is one of the co-authors of Brotli, WOFF2 and many image compression algorithms including JPEG XL.
[1] https://twitter.com/jyzg