That is correct. I hosted a board game day over the weekend that XRP came for and they had just gotten this book from the printer so they gave me a copy. They just can't sell any till October.
I'm trying to get them to do a book on art and culture. Of course my opening line to get them to do it was "I'm sure this idea will make you no money, but..."
There's a good 12+ pages of non-system dependent information in Plague that's very much in line with the Magical Society line's tone, but there's also a healthy chunk of 4e rules that can be used to bring Plague into your game. I think it's a good mix that should please most readers.