What they’re saying is that a web server can create a traditional jpeg file from a jpeg xl to send to a client as needed.
Other way around, you can convert a “web safe” JPEG file into a JXL one (and back again), but you can’t turn any random JXL file into a JPEG file.
But yeah, something like Lemmy could recompress uploaded JPEG images as JXL on the server, serving them at JXL to updated clients, and converting back to JPEG as needed, saving server storage and bandwidth with no quality loss.
Other way around, you can convert a “web safe” JPEG file into a JXL one (and back again), but you can’t turn any random JXL file into a JPEG file.
But yeah, something like Lemmy could recompress uploaded JPEG images as JXL on the server, serving them at JXL to updated clients, and converting back to JPEG as needed, saving server storage and bandwidth with no quality loss.