Hmm, I wonder where the line is drawn there. This isn't exactly a plot event that could be filmed. We're talking about a concept a character thinks to be true, wrongly so, and (without my own copy of The Silmarillion to hand) probably not even stated in dialogue in the original book. With the exception of Queen Beruthiel's cats, The Silmarillion provides the entire backstory for the other books. Surely the general concepts can't all be off-limits.
For example, the film Saruman's lesson about how the orcs were originally elves is not spelled out in the written LOTR. That's from The Silmarillion too.
Sure, you're right and I have no idea how it works in practice. I just think that it may be one among several factors that Peter Jackson has to consider when taking liberties with the novel, not that the Tolkien estate, as grumpy as it may be, seems overtly litigious.