Is there precedent for Apple using a completely different Intel part? I can't recall them using anything proprietary.
Afaik the first Macbook Airs used some custom Intel chip that (at first), was only available to Apple.
Semiaccurate had an article about Apple using a custom build Haswellchip for this generation, but (I never read it myself) it was said, that it was about some beefed up Iris graphics ...