Well, first of all, they are probably already booked solid but you can try.
Second, what they mean is you could have book it when you purchased your tickets, even before the 180 day mark. They've been doing that for party goers for years.
Third, the characters and cms there will be festive. The characters will have eye masks, or an apron that says Happy Halloween. The cms will wear make up to look like Haunted Mansion cms.
The food is the same as always.
Hurry and call if you are interested to see if they have any ADRs left.