Maybe only one small part of the movie was rendered this way. When they started the production there was not even a beta version ow Windows XP for AMD64 available and so I guess they just used their existing 64-bit Linux rendering farm to do the job. And I'm sure they would not risk such an important project to try to use some beta OS and beta apps. So they must have used WinXP for AMD64 just for some small and not so important part.