I don’t know. I was wondering myself. There are several possibilities I can see.
1) The Silmarillion and the press release just got things mixed up. Since it would be released after The Hobbit they think it’s a sequel when it’s really a prequel. (And I have no idea how they would make the Silmarillion into a movie, let alone more than one movie. It’s very convoluted and covers a whoooole lot of time.)
2) Expanding some story somewhere from the LotR appendices. (It’s been a while since I read them and I don’t distinctly remember anything in there from between The Hobbit and LotR, but maybe.)
3) Something from any of the various books published after his death that deal with the world. Again, “sequel” probably isn’t the correct word to use.
4) A completely made up Middle-earth story because, you know, this is a cash cow and people will pay to see it anyway. Especially if Peter Jackson is attached to it.