Second, as far as I can tell this is a JPEG encoding library, producing JPEG files that any existing JPEG decoder will be able to read. Nobody is being asked to support or maintain anything new.
Note that that's from almost a year ago, don't know if anything changed.
There's more to it, of course, but it's essentially just an improved encoder versus a new format.
This team did a lot of work to create JPEG XL. For whatever reason, Chrome is not agreeing to ship it, which will make it a lot harder for it to be widely adopted. So they're now applying the same techniques to classic JPEG, where the work they did can provide value immediately and not be subject to a pocket veto by Chrome.