Levistus's_Leviathan
5e Freelancer
No, I know that they're going to release the book on its own later on the year, I just don't think that they would do this sort of thing for the release of a book that we already know is coming.It's what they said in the stream so I don't know. I'll find the news page quote for you.
There's no reason that they would put a leak on Amazon saying "tune in on October 12th for a new book announcement!" for a book that they've already announced and told us about. I think that this book is going to be different from Monsters of the Multiverse. Probably Spelljammer/Planejammer, or something like that.