random guy said:
Yes, AMD will hold control of the API, but it will be free for anyone to implement
Meh, that is the least interesting part of "open" and AMD knows it.
You don't think Huddy knows perfectly well that no one in their right mind is going to sign up for an API designed by and for competitive hardware that could at any point change something that you can't support (efficiently or otherwise)? Of course he does, which is why there's no need to put up legal barriers. He can call it "pride" if he wants but in reality there are a lot of details (beyond how you map pixel shaders to SIMD lanes, ironically a fairly irrelevant point in terms of these APIs) that may or may not map well to various architectures and the Mantle design does not consider anything beyond GCN.
It's really this simple: you either care about portability and design for it and talk to other vendors or you don't. There's no half way in the middle where you design for your own product then later say "well maybe if you mess with it a bit you could kind of support it a bit efficiently, I guess?". That's pure PR. AMD could support Intel's register specs at the GPU interface level too (or vice versa) - it's all fully documented. Do both companies just have too much "pride" to do it? It's a silly argument.