Myrhn is on the right track - 'hearing' is actually the term used to describe CNJs who are especially sensitive to the whisperings of the elementals, and thus are highly regarded in Gridanian society. Hence these talented individuals are called hearers. Sylphie might have a WHM-esque talent for healing, but as it turns out, once she finally let go of her stubborn single-mindedness, she was revealed to be highly sensitive to the words of the elementals, which would allow her to train as a hearer - in no small part thanks to the player's gentle nudging her in that direction.

So yeah, there is no mistake in that line.