Not necessarily. Zone of Truth allows the speaker to be willfully evasive on a failed save. Eloelle just has to start giving faerie-style answers to the questions. (I mean, she IS a genius.)
The answer posited was not evasive- "I don't know the answer" is a bald-faced lie.
Alternatively, Eloelle's patron clouds her mind, to ensure that this arcane truth does not fall into the hands of this sorcerer. The patron has plans for this information, of which Eloelle is just starting to become aware...
Again, giving the character an advantage that isn't reflected by the mechanics.