They did the same thing with Pearl Bailey as Big Mama in The Fox and the Hound. In her case they actually animated Big Mama to Pearl doing one of her songs. Still it's obvious why the World need good animators.
What's cool is the animators clearly took some cues from the actors but didn't Bakshi the thing and put their own spin on the movements. Makes sense that they'd use the same audio though if they wanted to keep some semblance of the original actors' energy on the final film.