Pixar uses a multitude of software and systems for its animations. Its likely they use both the new Mac Pro as well as Linux.
http://www.businessinsider.com/pixar-uses-apples-mac-pro-to-make-films-2014-1
Random posts from some website is not definitive proof of what they use.
The guy, Craig Good who replied and commented whether Pixar uses the New Mac Pro, is a filmmaker, writer and voice actor has done some work with Pixar Movies like Toy Story, Brave etc. Likely he has an idea on the setup Pixar Studios uses.
http://www.imdb.com/name/nm0328678/
http://www.craig-good.com/movies