I agree that her writing (and her performance) was astounding, especially for someone so young.
I also agree that writing poetry and lyrics, while seeming to be very similar, pose different challenges that the writer needs to have an awareness of if they're going to be successful. But that doesn't make it impossible.
Sondheim writes about the difference between lyrics and poetry in Finishing the Hat. As you'd expect, it's very insightful and well written.