The web site is wrong. The restaurant is on the official plan brochure, and we've had no reports of it not accepting the plan. But this is only one of many errors when it comes to dining plan information on the online booking engine - the number of credits is wrong for Le Cellier on the deluxe plan, several restaurants show as accepting either the basic plan but not deluxe or vice versa, and more errors seem to pop up (or get discovered/questioned) on a regular basis.
Right now, the safest thing to do is go by the plan .pdf on the Disney site, because the restaurant information/reservations section is a train wreck.