Yeah, i guess they (norton) don't want their ad copy to tell you the whole story on it, or you might not buy it!!
It was, at the time I bought mine, about $25, and well known among the woodworkers that it was only good for soft woodworking stones like King and Norton, but the fact that they weren't always flat and they don't stay flat forever drove a lot of people nutty.
For an old school woodworker not afraid to flatten their flattening stone on a sidewalk or driveway, and with a bucket full of king stones, it'd probably be fine. Otherwise obsoleted by diamond plates, though.