The quality of the preview layer is somewhat under the control of the user though. And that comparison was made for a JPEG-XL focused site, and uses quite larg

The quality of the preview layer is somewhat under the control of the user though. And that comparison was made for a JPEG-XL focused site, and uses quite large images that are then scaled down dramatically (at least on mobile) for display which is probably not an ideal visual test anyway (but possibly necessary for there to be enough time to see any difference)