I guess the debate then comes down to if Apple, as an insider, had already committed to development of Metal before they knew AMD was up to something.
With the public announcement of Mantle in Q3 2013, and the first release of Metal in Q2 2014. (With the first release of Mantle in Q4 2014 afaik). I still think there is enough slack in the timelines given that Apple must have had inside information. But I could be very wrong of course.