The online system is full of bugs. Common sense should tell you that Coral Reef would not be open for dinner for a 30 minute period when the other Epcot restaurants are open. Don't worry about it.
To give you an idea about the online wacky system, it would never show any availability for Sanaa for dinner. (Not sure if that's still the case, but it was definitely an ongoing online bug.) But there is usually good availability there since it's a restaurant off the radar of most people.
Your best bet when you encounter weird online stuff is to call the dining line.