You are right. I checked the Zaentz website and it is pretty much everything *BUT* the Literary rights, but from what I've read about prior LotR games, they still had to negotiate terms with the Tolkien estate as well. One game found itself with the requirement to include a copy of the first book with their game.
But that wasn't creating _new_ adventures like the propose manga is.
If the manga is proposing a manga telling of LotR, that could happen as long as there are no deviations in story or abridgements. In which case, I wish them luck! That should take like 10 years or so to complete.