One could also argue that a renegade Eelfinn gave Elza this knowledge, or something like that, simply because this statement doesn't preclude it. The point is that nothing suggests that Graendal was involved, and since this idea actually differs from what we see in the book, namely only SH and Elza present, one has to give evidence for this Graendal-idea if one thinks it's that likely. As said, also all of BS's other quotes on this topic indicate that only SH and Elza were involved, because only they are mentioned.