Whats the hardware? How much electric energy goes into rendering a frame or a whole movie? How do they provision it (as they keep #cores fixed)? They only talk about cores, do they even use GPUs? What's running on the machines? What did they optimize lately?
So many questions! Maybe someone from Pixar's systems department is reading this :)?
From my experience (animated movies) GPU is still very experimental because of how limited it can scale, and definitely not use in render farm.
And I'm not even talking about the cost.
GPU rendering demos focus on speed, but of the biggest problem with full feature is flexibility. The more complex your image is, the more problems/artifacts you will "create" on it. Your render time can be x100 faster, if you need to spend two days to fix a problem for each shot, the quality VS speed ratio completely fall over.
Everything get easier, slowly, so maybe one day we will have a 100% GPU farm on big budget projects, but for now, CPU is the most predictable way to manage large scale rendering for both sides (sysadmin/artists).